Historically it makes more sense to start continental and then be pushed to the "fringes" of Europe by Rome and then the rising medieval European powers. Spawn in France, spread to Britain and Ireland (and Spain?) before being driven out by superior forces and flips. UHV goals based on Culture, maybe techs (first to metal casting?) and expansion. End up as a minor power in Ireland, barring outrageous luck or player intervention.
1st: The major problem is balancing with gameplay. If we let spawn in continental Europe it will be disastrous for the reasons I mentioned above. If we let them spawn in England, then the game will balance.
They will manage to get control over British iles, in ancient times they will interact basically with romans. Moreoever, they will propably send some settlers in northern France, in northern Spain, it is way much more historical. If they spawn in continental Europe, the French and Germans will face a celtic empire, which is completely unhistorical.
In medieval ages they will be there in Viking spawn, thirsty to start their raids as soon as possible. England will have to conquer Scotland and Ireland.
2nd: Historically makes more sense. The Hallstatt culture is a prehistoric culture, like LaTene, Bantu, Protosiberian and Slavs and many-many cultures that existed throughout the world which aren't represented in the game. The only place that they formed political unities was in british iles.
Let them spawn in England with three galleys. Make them expansive, so they will expand in France and Spain. The barbarian invasions, the roman imperialism and the viking raids will form a much more historical and realistic scenary.
Let's propose a UHV:
The celts:
Spawn at England in 500BC:
UB: Dum or Menir: replaces pagan temple, +2

UU: Galic warrior: 5STR,free hillsman promotion, +10% against cities, replaces axeman, it doesn't require bronze.
UP: The power of dryism: forests cause +1

.
UHV:
*Control Wales/Cornwell, Scotland, Ireland, Britagne, France (2 cities), Northern Spain by 200BC
*Control 10 pagan temples in 200BC
*Capture Roman capital.
AI will spawn with 3 galleys. French cities will spawn normally.
Humans spawn with only Mysticism, Agriculture, Mining, Bronze Working, the Weel, Fishing, Sailing, Masonry and no galleys.