The UK was, in terms of their army, no threat to Germany, and by early 1941 even their airforce was little threat. Yes, Germany was having a tough time knocking out the RAF in southern England but this was due to the English defensive strenghts and the concomitant German weaknesses in offence over England. Most of the British planes were short range fighters that were excellent defensively but could do little to bring the war to Germany. Only after Germany turned its attention, including most of its Luftwaffe, to the east did the British get a chance to build up their air force, including heavy bombers that could start bothering Germany in a major way. But let us not forget that this recovery of British strength was due in large part to supplies coming in from Britain's allies, including the US, which was in the war by this point in all but name.

Germany launched Barbarossa with the intention of defeating Russia in a few months. The Germans never planned on waging a long, drag-out war with the Soviet Union. If the Germans knew that they weren't going to defeat the Russians quickly I doubt they would have attacked when they did. Now, Germany's failure to anticipate a long war with Russia, one they would not win very quickly in typical Blitzkrieg fashion, can certainly be held against them. The Germans figured they could leave a broken Britain in their back while attacking Russia because they figured it would only take a short while to defeat Russia, and at the conclusion of this short, victorious war they could once again swing their Luftwaffe and other forces back to face England before England could recover her strength and pose any real threat to Germany's "Fortress Europe". Only when Germany couldn't extricate itself quickly after a short victorious campaign in Russia did Britain (and the real strength behind them, the US) become a serious problem for Germany.