Before the Great War, the United Kingdom was made up of four countries: England, Scotland, Wales and Northern Ireland. The UK also controlled many overseas territories. During the post-Divergence years, the UK was the leader of the states in both the Commonwealth of Nations and the European Commonwealth. No details are known about the state of the British Isles after the Great War.
"Allistair Tenpenny came to the Capital Wasteland from Great Britain to seek his fortune, so that alone tells you that the U.K. was also hit in the war. And if he came to U.S. to succeed, that says a lot about how screwed up Europe must be. So we just allude, a little bit, to the state of the rest of the world. We like to leave a lot to the players' imaginations, and somebody like Tenpenny serves as a catalyst for those thoughts." - Emil Pagliarulo