Common thought, but you can never be sure. We still had those, although exhausted 300,000 British troops ready, not mention a lot of fresh faces not including 'Dads Army'. The Royal Navy was still the most powerful, 168 Ships, including subs which still blockaded the North Sea, throughout the war.
Would have been quick to mine the Channel, even more than the Germans did in 1939 - 1940. And even then, with experiences the Allies encountered in 1944 it wouldn't have been a walk in the park. Also, Hitler, nor the Nazis had any plan for making us speak German.
Also, Churchill had prepared the most well organised resistance movement had we been taken, better than the French, Norweigan and Polish.