OscarWildebeest
Prince
England/Britain is one of the hardest civs to represent sensibly given the numerous waves of peoples that formed it. However I will make them have Celtic origins in this mod for 5 reasons: 1) The Britons were the first defined civilization (that we know of) on the isle, 2) in my opinion the Britons were quite distinct from the continental Celts, at least as much as the Germans-Vikings or Spain-Portugal, 3) Germanic peoples are already being well represented in Germany, the Vikings, France and later periods of Britain, 4) I have some Welsh ancestry so I'm somewhat biased, and 5) there is extremely little quality Anglo-Saxon unit art available but plenty of Celtic.
Being Welsh myself, I have a vested interest in this bit. The problem is exactly as you define it - here in the UK we're a mixture of native, Roman, Norman, Anglo-Saxon and Danish (and that's not taking account of recent immigration from the former parts of Victoria's Empire). Even the definition of 'Celt' varies according to, well, the mood and agenda of the person using it.
The modern Welsh are descended from the indigenous Britons who mixed with the invading Romans in the first century BC, and who were driven west by the invading Normans in the eleventh century AD. Since then there has been a great deal more mixing with the English (too much, some would say ) so that it's impossible to say who among us today is descended uniquely from those original inhabitants. I know my great-grandmother's family were farming the land several centuries ago, but since she was called Jones it's a bit difficult to separate her family tree from the millions of other Joneses (and 'Jones' is clearly a name of English origin, since there is no J in the Welsh alphabet).