Can't help thinking that, assuming the Nazis had got ashore in sufficient numbers, and were able to maintain their supply routes, they would simply have been slowed down. After all, they simply avoided the Maginot line, and went straight through the Ardennes instead.
I think if the Nazis really wanted to invade, they would have built a fleet of proper landing craft. That they never did says a lot. Given air superiority and U fleet support, I think the result would have been a short, intense battle, occupation of London, some sort of uneasy coming to terms, and probably years of guerilla warfare from the north of the country, until eventually the Americans and/or Soviets chose to liberate us.
Whatever, it never happened. In some ways, Hitler and his refusal to listen to his generals was our best asset.