Well, a lot of people seem to be in agreement that the movie is definitely attacking something, whether it be the US or the UK.
Actually, it could be just about any country. The movie clearly shows that totalitarian dictatorships rise because of the peoples' trust in one individual to pull them through a crisis, and through fear of outside threat, they grant the leader more and more power to the point that the leader is a dictator.
This really could be a reference to anyone: the Germans post-WWI, WWI-era Russia, post-9/11 America, the UK after the 7/7 bombings, and (to some extent) Spain after the Madrid bombings. The fact is that the movie is more centered on the UK and the US because those two nations are at the most risk for giving a leader too much power as a response to fear.