At the time of Barbarossa, everything that Germany could muster was sent to the east. All that was left in Western Europe was a skeleton force. You can not simply leave a country like France without a garrison. GB would definitely need one. Even if you take a few divisions from France and transfer them to GB it doesn't solve Germany's dilemma in the East. Italy could have sent an additional 30 divisions and it wouldn't have made a difference in Russia. The Greeks beat them and so did the outnumbered British troops, Russia was a different animal. Capitulation of GB for example also didn't guarantee all of British forces surrendering. France is a good example. Small group of pilots volunteered to fight alongside the Russians in Leningrad for example.... Why should it be any different with the British lads?