The mainstream thought is that of course, England won WW2. But Great Britain began to lose many of her colonies and stopped being a great empire. Her best navy in the world became a shell of its former self. She ended up on American food stamps.
Did Great Britain gain from World War 2 or did they lose? Would Britain have been better off not declaring war on Germany?
You seem to be suggesting that it was WW2 that led to Britain losing its empire, Britain, however, was already moving towards granting independence to its colonial possessions before WW2. If anything, WW2 delayed the break up of the empire. Arguably Britain would have been better off if an accommodation with Germany could have been reached. However, the view at the time was that, even if that were the case, a case that was put at the time, Britain could not allow Germany to continue with its aggression.
The break up of Empire and the creation of the Commonwealth wasn't merely the result of economic and geo-political pressures. The Labour government that was elected in '45 did not subscribe to the imperialist mind-set of the War time Tory-Liberal coalition. When democracies you can't discount elections and their powers to change the course of events.