I'll land in Dread Knot's corner. Just as Germany had nothing to gain with a war against the US, it also shared much and had much to lose by going to war with Britain. So long as they kept the British and Americans out of the war, owning the entire continent and Russia too was well within their capabilities.
I believe that even as late as Dunkirk, a rational Germany could have played nicey-nice, saying that they allowed the British to escape for humanitarian reasons and that they had no quarrel with the British people. They could have offered and held to a non-aggression treaty with Britain, granting them most valued trade status. I don't believe Britain's obligation to Poland would have justified hundreds of thousands of casualties if they continued at war.
This would have left Germany with a secure continent, actually protected and secured by the British, leaving them full access to all of their military means to attack Russia. They wouldn't have needed their subs any longer. A 100% effort would be focused eastward.
That would have left Russia, a communist country with no real friends, to be roasted like a Thanksgiving turkey.
After the war, we would have learned that there were much worse things to deal with than Soviet Russia...
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