Nice idea for a mod
Here's some info on British tribes I found on the internet.
*Belgae The Belgae were a warlike people of ancient Northern Gaul, separated from the Celtae of Gallia Lugdunensis by the rivers Matrona (Marne) and Sequana (Seine). According to Strabo the country of the Belgae extended from the Rhenus (Rhine) to the Liger (Loire). In the opening passage of Caesar's Gallic Wars, the Belgae are described as forming "a third part of Gaul". Belgica was one of the four provinces of Gaul near the Rhine, delineated by Augustus. The British Belgae no doubt descended from a Belgic colony.
*Brigantes The Brigantes were a tribe - or perhaps more accurately a loose confederation of related tribes - of British Celts inhabiting almost all of the area between the Humber and the Tyne. The name of the tribe springs from the Celtic goddess Brigantia.
Brigantine society was primarily pastoral, in contrast with their southern Celtic neighbors. They used pots and bowls of wood and leather, though they certainly had the technological know-how to produce pottery.
At the time of the Roman invasion in 43 AD the Brigantes were arguably the most powerful Celtic tribe in Britain. Initially the Brigantes, under the lead of their queen, Cartimandua, were on friendly terms with the Romans, acting as a "client-kingdom".
In fact, it was Cartimandua who betrayed Caratacus to the Romans, thus depriving Celtic Britain of its most influential and steadfast resistance leader. It was also a fairly typical act for the Celts, who could never stop fighting amongst themselves long enough to mount a serious obstacle to the Roman advance.
Cartimandua had cause to be grateful to her Roman allies; in 57 AD her husband Venutius tried to sieze power, but the Romans put down the rebellion. The couple were reconciled for a time and ruled jointly until 69 AD, when Cartimandua deserted Venetius for his armour-bearer, Vellocatus. She eventually fled Brigantine territory and was never heard from again.
In 73 AD the governor Petillius Cerialis invaded, and defeated Venutius, but continued unrest led to Agricola finally annexing Brigantine territory for good in 79 AD. Isurium (Aldborough, near Ripon) emerged as the administrative centre of Brigantine territory, though there is no evidence to suggest any settlement there prior to the Roman invasion.
But the Brigantines were not finished; in 138 AD, when you would be forgiven for thinking that they must have been thoroughly "Romanised", they rebelled against the Emperor Antonius, who was attempting to push north from Hadrian's Wall into Brigantine territory in modern Scotland. The Romans under Lollius Urbicus quickly put down the revolt. In 154 AD the Brigantes rebelled yet again, with similar results.
Some archaeological evidence suggests that there may have been a Brigantine presence in Ireland; certainly a second century map by Ptolemy shows the Brigantes there, and excavations on the island of Lambay show Brigantine artifacts dating from the end of the first century AD. This might indicate a settlement of Brigantines fleeing from the final Roman occupation of their tribal territories in England.
*Caledonii This is the name of peoples who lived in the Scottish Highlands and Islands. The Romans used the word Caledones to describe both a single tribe who lived in the Great Glen between the modern towns of Inverness and Fort William. They also called all the tribes living in the north Caledonians. We know the names of some of these other tribes. They include the Cornovii and Smertae who probably lived in Caithness, the Caereni who lived in the far west of the Highlands, the Carnonacae and the Creones in the Western Highlands.
The Vacomagi lived in and around the Cairngorns. Other unknown tribes lived in Orkney, Shetland and the Hebrides. Warriors from many of these tribes came together to resist the Romans under a leader called Calgacus at battle of Mons Graupius in AD 84. Although the Romans won this battle, they never successfully conquered the Highlands. The Romans admired the Caledonii for their ability to endure cold, hunger and hardship. Tacitus described them as red-haired and large-limbed.
Unlike the Taexali and Venicones, the Caledones rarely made religious offerings of fine metal objects.
*Catuvellauni The main territorial ground of the Catuvellauni was to the north of Thamesis (River Thames), and to the north (modern Hertfordshire) and west (up to modern Oxfordshire) of that. They were one of the most prominent Celtic tribes of their time, and also one of the richest. Nevertheless, as with all the pre-Roman Celts, they left no written records. Their rulers are only noted after they came into contact with the Romans
Cruithni It is clear from Irish sources that the Picts were known to the non-Roman world by another name..."Cruithni or Cruithnigh." When we consider that (1) the Pictones were ancient friends of Rome during the reign of Julius Caesar, (2) broch people were called Cruithni in Galloway and Argyll, (3) Cruithni is the native Celtic name for Pict, and (4) the broch culture slowly entered Scotland completely vanishing from its former existence in the west and north, then it appears logical to equate the mysterious broch culture with the Picts. If this theory is correct, many difficulties are erased. The Pictish language is not a mystery. It was a form of Gaelic and surviving Welsh. The desertion of the brochs is explained by the Picts moving south into the devastated, although richer, lands of eastern Scotland when the Roman armies withdrew to Hadrian's Wall. This moved the Pict central kingdom into the richest district of Scotland. The Picts fought Rome for the first time in many generations. At the outcome, all people who raided the British provinces in the late years of the Roman Occupation became known as Picts.
According to the Celts, the Picts were also the Cruithni who came from the north and conquered one-third of Britain. It seems to be an inevitable conclusion that they came from the brochs of the Orkneys and Shetlands and that one-third of their land included the former territory of the Macatae in Strathmore where the later Pict kingdom was centralized. This is a drastic suggestion, and it is necessary for me to discuss this in detail. The late Professor H. M. Chadwick believed the Picts were the people of the Gallic-Wall forts at the time of Agricola, approximately 300 years before his time. The Caledonii are shown in Ptolemy's geography as holding the central mountain mass of the Scottish Highlands. The Maeatae were believed to be the people living in Mag More (Great Plain), which is Strathmore today. It is said that the Cruithni took the Mag More by the sword.
If all the above is correct, the Picts were the same people known as Caesar's allies on the coast of the Bay of Biscay. They had moved into the Hebrides, Orkneys and Shetlands following the defeat of the Veneti in 56 B.C. They expanded at the expense of the older Celtic tribes into the lands of Sutherland, Caithness, Ross and Cromarty. If Severus had pursued his advance as far as the scanty records of his campaigns indicated, he probably had contact with the Broch men ...Picts ...Cruithni. Although this is speculation, it appears that the Broch Men must have always been in opposition with the Gallic Wall tribes. Why else were the brochs constructed? Rome was too far south to be dangerous. The brochs possibly were built in anticipation of a lengthy counter attack. History sets forth the premise that for generations the Romans and Broch Men hunted in pairs.
*The Damnonii, Novantae, Selgovae and Votadini occupied, what would today be called, southern Scotland. Damnonii can also be spelled as Dumnonii, but there is no reason to suspect a connection between this tribe and the Dumnonii of south-western Britain. The Selgovae (their name is thought to mean 'hunters') were possibly a part of the Votadini - a large federation of smaller tribal groups. Around AD80, Agricola began his campaigns in Scotland. These southern tribes were conquered, and, in c.AD84 at Mons Graupius, Agricola also defeated a coalition of northern tribes - Caledonians. However, as early as AD86, the Romans began a gradual retreat south, arriving at the Tyne-Solway line about AD100. The Tyne-Solway frontier was formalised by the construction of Hadrian's Wall - ordered by the emperor Hadrian, on a visit to Britain in AD122. On the instructions of emperor Antoninus Pius (ruled 138-161), the Romans, once more, marched north. They built the Antonine Wall across the Forth-Clyde isthmus in the early AD140s, and the southern Scottish tribes were occupied for a further twenty odd years. In about AD164, the Romans, again, withdrew to Hadrian's Wall. In AD208, Septimus Severus began a campaign in Caledonia, but, after his death (in AD211 at York), Hadrian's Wall once more, and finally, became the frontier of the Empire. Although they were beyond the frontier of Rome, the tribes of southern Scotland were not beyond the influence of Rome. Several outposts to the north of Hadrian's Wall were garrisoned - the area effectively becoming a buffer zone between the northern barbarians and civilization.
*The Dumnonii is the Latin name for a Celtic tribe which emerged in the wake of the Roman withdrawal from England during the 4th century AD.
Their territory spanned Devon and Cornwall with further holdings in Somerset and Dorset. They were possibly the most powerful of all the British Celtic tribes. It is also possible that the Dumnonii held some Breton land at some point. The name of Dumnonia in Celtic is Dunein.
Originally their capital would have been at Isca (Exeter), although archaeological evidence indicates that Celtic occupation ended during the 5th century AD, no doubt as a result of the expansion of the West Saxons.
Dumnonia was effectively a collection of sub-kingdoms.
Durotriges Occupying the south-west peninsula, the Dumnonii were probably a grouping of small tribes. They did not use coins or have any large settlements. They seem to have offered no resistance to the Romans, and as a result few garrison forts were placed there. They did not fully adopt the Roman way of life, and carried on in their own style throughout the occupation. The Romans established Isca (Exeter) as the capital of the civitas.
*Iceni The Iceni were a tribe of British Celts living in the area of modern Norfolk and north-west Suffolk. After the Roman invasion they retained their territory as a client kingdom. In 47 AD the Iceni rose in revolt after the Romans tried to enforce a law forbidding the carrying of weapons. The revolt was put down and Prasagustas established as a client king. Prasutagas decided that it would be prudent to make his will assigning half of his personal property to the Roman emperor. When he died in 61 AD the Roman officials decided to interpret his will as a submission to the Roman state, so they moved to appropriate all of the Iceni lands and disarm the tribe. Not surprisingly, Prasutagas's widow, Boudicca (or Boadicea as she is sometimes known) protested. The Romans had her publicly flogged and her two daughters were raped. This high handed treatment of an ostensible ally had predictable results. In 61 AD Boudicca raised the Iceni and the neighboring Trinivantes tribe in revolt against Roman rule. It was only with some difficulty that the Romans managed to quell the Iceni uprising, and Boudicca took her own life rather than fall into the hands of her enemies. The Romans established a new capital in the Iceni lands at Caistor-by-Norwich. No record of the Iceni exists after this point, though legends suggest that large numbers fled west into Wales and eventually settled in Ireland. The known and speculative rulers of the Iceni are as follows:
Prasutagas ? > 61 AD
Boudicca ? > 63 AD
*Ordovices Celtic tribe located in the north western part of Wales. Famous for their total resistance to the Roman occupation. This tribe despite all attempts by the Romans at subjugation, managed to retain their freedom.
Silures a powerful and warlike tribe in ancient Britain, occupying approximately the counties of Monmouth, Brecon and Glamorgan. They made a fierce resistance to the Roman conquest about A.D. 48, but a legionary fortress (Isca Silurum, Ceerleon) was planted in their midst and by A.D. 78 they were overcome. Their town Venta Silurum (Cmrwent, 6 m. W. of Chepstow) became a Romanized town, not unlike Silchester, but smaller. Its massive Roman walls still survive, and recent excavations have revealed a town hall and market square, a temple, baths, amphitheatre, and many comfortable houses with mosaics, &c. An inscription shows that under the Roman Empire it was the chef-lieu of the Silures, whose ordo or county council provided for the local government of the district.
*Trinovantes In the period immediately preceeding the Roman invasion of Britain the Trinovantes tribe occupied the area of Britain now taken up by Essex. The "capital" of the Trinovantes may have been at Colchester (Camulodunum).
There is some suggestion that the Trinovantes rulers may have been "under-kings" of the Catevellauni. The expulsion of a Trinovantes prince by Cassevellaunus of the Catevellauni was the pretext for Caesar's invasion in 55BCE. The Trinovantes saw the Roman invasion as an opportunity to strike back at their northern rivals and they were quick to join with Caesar and strike against the might of the Catevellauni.
After the more permanent Roman invasion under Claudius in 43AD, Colchester became a colonia (49AD), or Roman city of the highest rank. The Trinovantes enjoyed the fruits of cooperation with the Romans until the loss of tribal territory to Roman settlers caused the Trinovantes to join with the Iceni revolt under Boudicca in 61AD, and the colonia of Camulodunum was burned to the ground.
The Trinovantes vanished from history after the failure of the Boudiccan revolt until medieval romances linked the tribe with the names of the legendary figures Brutus and Corineus, mythical founders of Britain and Cornwall, respectively.
Known and possible kings of the Trinovantes include:
Addedomeros
Dubnovellaunos
for this remark ...I'm surprised to see so much 
