Britain and the UK are the same.
And there is nothing remotely welsh, irish or scottish about it well I think they sell our flags.
xD
I disagree with you on this one

have you read The Imagineering Guide to Epcot? Definately some Welsh, Nrth Ireland and Scotland influence on the pavillion. It obviously has British aspects about it, you could easily argue there are no obvious individual aspects for Scotland, Northern Ireland, Wales and yes, even England. You can disagree, but after reading about it I found that theres lots of architectual (god knows how to spell that!) influences from all countries in the UK.
For me, most likely Russia. I saw the plans for it and yeah, looks awesome. Its been rumoured to be built for ages. Greece would be cool, but I am confused when people say Africa pavillion, Africa isn't a country it's a continent.. Egypt is in Africa, and Animal Kingdom is themed after both Africa and Asia..
So overall, Russia and Greece

I s'pose Spain would be cool too..