Since we're talking about it the British front so much, let me put forward an opinion, and ya'll can correct it if I'm farting rainbows:
If Hitler had really, really wanted to take down the Island, if he had devoted his armies to that task, rather than half hearted attempts to break the British will to fight, he probably could have launched an invasion that stood a decent chance of success. Of course, this would open him up in the East for Stalin to come on in, but the point is, the British were saved, IMO, by a combination of their own skill and gumption, and Hitler's unwillingness to commit necessary resources to really preparing for a successful invasion.
But I could be wrong, and I welcome correction if my opinion is based on faulty assumptions.