Great Britain

The United Kingdom (possibly the British Commonwealth) is the only confirmed member of the European Commonwealth.

In real life, the United Kingdom is a country made up of the four constituent nations of England, Scotland, Wales and Northern Ireland.

An island nation in Europe far to the east of the core region/known world for the Fallout series. Before the Great War it was a wealthy country and most likely a nuclear power (like its real world counterpart). It was the leader of the states in both the normal Commonwealth and European Commonwealth. Allistair Tenpenny is known to have come from the United Kingdom before traveling across the Atlantic. No details are known about the state of the British Isles after the Great War.

There are some historical references to the United Kingdom in Fallout 3. The Declaration of Independence can be found which historically marked America's independence from the British. The Magna Carta (which was an important document written in England in medieval times) can also be found. Scotch also originates in the United Kingdom (it is a beverage originally brewed in Scotland).

Colonial Empire
Britain, as with it's european neighbours, once had a colonial empire, even for a short time after WWII, with cities like Hong Kong,under their control, but with China's aggressive expansionism, and possible invasion of India (it is unsure if India ever did gain independence) may have caused serious clashes between Britain and China.

However with the nuclear holocaust Britain's control over it's imperial possessions if it retained them in the alternative timeline is likely to be between slim or nil.

Official statements

 * "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