Why after D-Day failed?

I've constantly amazed at how people believe that if D-Day had failed, Nazis would occupy Britain... It's not the first instance I've seen this bullsh**, either about Britain occupied after it or Europe remaining under Nazi control... as if Reds weren't advancing to Germany, as if Italy wasn't being occupied (or liberated)... There's no possible way Germans would be able to carry ou their own D-Day operation in mid 1944... they were in deep sh** by that moment with other things and it was just a matter of time until they'd be defeated...

Why wouldn't they start the movie with Briatain losing in the Battle of Britain, for example... or, if they wanted it to be closer to the end of war, why not 1943 when, for example, Germans decided to try to invade Britain again as a way of boosting morale after recend defeats in other fronts...