If Germany had conquered Great Britain, the Germans would have effectively won all they wanted out of the Second World War.

First and foremost, the loss of Britain meant the elimination of the United States as a threat because to launch an invasion three-thousand miles over the Atlantic that would be big enough to matter was simply not practical. Besides, America really had no beef with Hitler as long as he respected American commercial interests,

The British Empiresimply could not afford to carry on the war without Great Britain as a staging ground and political focal point. That is assuming that the Empire even remained loyal with the head decapitated. Many crucial parts of the Empire, like Egypt and India, probable would have broken away if Great Britain was forced to do a Free French routine and pretend to have a functioning government outside of the home nation. The ANZACs probably would have reamined friends but what good could 50,000,000 people separated by thousands of miles of ocean to do the combined forces of Western Europe under Nazi control? U-Boats alone would render communications beyond reason.

Finally, partisans only fight when they feel that there is hope. The French resistance fought on because it knew that it was only a matter of time before liberators crossed the channel. Without the certain knowledge of a liberation, partisans become less and less effective over time.

Last, a Germany free to use the resources of Europe as it wished, without a Royal Navy to blockade it, could easily overwhelm the Soviet Union. Hell, The Germans would not need to conquer the Caucausus oil fields because they would not have 750,000 British and Commonwealth troops backed by the largest navy in the world between them and Mesopotamia or the Dutch East Indies. Considering how successful the Germans were using the limited resources of Europe available to them in reality, it is not a stretch to imagine how much they could have done without the British stop sign at every corner of the continent.

Germany was ultimately defeated because the British divided German efforts enough to prevent it from overwhelming the Soviet Union. The Royal Navy effectively blockaded Germany from all sides apart from the Eastern Front. Great Britain allowed the bombers to slowly bleed the German warmachine to death while the full strength of the United States slowly massed for Overlord. Without the British Isles, victory was impossible.