Well Im British and I do mind Britain not being in the game, I have relatives from all over the UK not just Eng. I cant be bothered to change it and only just know how to do it anyway!
Nationalism is artificial in one sense in that its invented but in the modern world 'nation states' make up territory with the inhabitants of those states generally having a feeling of belonging to a nation, this is certainly on the wane in Britain with the rise of 'Nationalism' within all three countries, but the geographic terms the island is 'Britain' and the people are 'British'.
I know what would be cool though, a scenario in dark age Britain with Wessex, Picts, Vikings, Scots, Cornish, Celts, Etc. is there one?