I was searching online for more information about the decline of The British Empire. The huge amount of men from Britain who died in the first world war surely had an impact as well as the amount of money needed to run the war. However after the Second World war most of Britain's colonies soon gained autonomy afterwards. My question is this, which war really set forth the motion of The British Empire beginning to decline? And if neither war happened could it possibly still exist? Thanks.
Well, if you mean the overseas territories that constituted the Empire, it's likely that the Empire would have eventually disintegrated regardless of how costly the wars were. Britain's abuses in India and Ireland, the growing autonomy of the Dominions like Australia, Canada, and South Africa, and the inherently temporary nature of the Mandates in the Middle East -arguably- pointed to an inevitable collapse.
Looking specifically at World War I, Britain (and France) actually gained territory from the collapse of the Ottoman Empire by being granted "mandates" in Palestine, Syria, Jordan, and Iraq, as well as former German colonies in Africa and the Pacific. At the same time, however, India was agitating for independence and the Irish fought their way to a Free State. Canada, Australia, and South Africa were all rather nebulous concerning how much authority Britain actually exerted over them (especially evidenced by South Africa's reluctance to join World War II on the Allied side).
By the time World War II ended, Britain was obviously bankrupt, but what's more important in my opinion is the rise of the Americans and Soviets as the new major powers, neither of which fit the "traditional" vision of the classical European empire. (colonies, dominions, etc) The Americans were (and are) ostensibly anti-imperialist (direct imperialism, I mean), as were the Soviets (again, direct, traditional definition). So Britain found herself with a vast, multinational realm of unwilling (or de facto independent already) subjects that she was unable to pay for or defend, with the two emergent superpowers "encouraging" decolonization. Unhappy and externally oppressed nations were fertile ground for communism, and a British imperial exit could mean and American financial entrance. Britain (and France's) power projection could be quickly checked and curtailed by either of these powers, as evidenced by the response to the Suez Crisis. I can sit down and source this if needed, but I'm hoping others with more expertise will buttress my answer with more specifics and examples.