Fallout setting is a 50's America theme, if it were outside of America it wouldn't be Fallout so to speak. The devs said they won't take it out of America because of that. But you never know......
Well what constitutes '50s America'. Is it Eisenhower as President? Race Marches? The Korean War? The Mickey Mouse Club? Hopefulness and Advancement? Science fiction? We had similar stuff happening OUTSIDE of the U.S. The first major Television broadcast to be watched by millions of people at the same time was Queen Elizabeth's Coronation in 1952, not to mention that despite rationing the U.K. had an air of hopeful joy of seeing though the last war and we had Sci-fy in the form of characters like Dan Dare pilot of the future and Victor comics began publishing, we also had our problems with youth and there were biker gangs and 'Teddy-boys' on the streets, there was also the beginning of the Welfare state. A lot of interesting things happened outside America during the 50s, and quite frankly considering that this is alternative history the devs could do whatever they damn well want.
Also it was the last great age of British Jet design, such as the Lighting, http://upload.wikimedia.org/wikipedia/commons/2/2d/Lightning.inflight.arp.750pix.jpg and the Vampire http://farm4.static.flickr.com/3137/2770279344_62a502c7b3_z.jpg?zz=1 after all, we did invent the jet engine.