Would have been tough in any case...he had to get across the Channel...best shot at that was very soon after Dunkirk IMHO.
But several million men were thrown at Russia...if he'd taken two million of those and applied them in the west...who knows?
I'm glad the maniac split his efforts. It let us beat him all the quicker...and that was a tough nut as it was.
I think even Hitler after 1940 realized a cross channel invasion would be very difficult. Even if the Luftwaffe had established air superiority over Britain it would have been very tough. They probably would have focused on the u-boat blockade and attacking the British in the Mediterranean.