RE: Britain might well have been better off making peace and sitting on the sidelines munching popcorn whilst Watching the Nazis and the Communists slug it out indefinitely
You mean to say Hitler had absolutely no interest in conquering Britain at all?
No, not really. He made clear in Mein Kampf that he had great admiration for Britain and its Empire. He was furious and disappointed when he found himself at war with Britain over Poland. He might have wanted a war with France in the long-term because of Alscace and Lorraine, but he had no particular beef with Britain.
Britain could have sat this out to its own advantage, but didn’t. It is debateable how advantageous it was for the world to defeat Hitler, considering how evil Stalin was, and how many people were murdered in the name of Communism, the scale of which dwarfed the atrocities of the Nazi Holocaust by a considerable margin.