[Laurent Rizzotti]

The biggest difficulty of a German invasion of Britain in 1940 would be to bring enough troops to defeat the British Army, even weakened by Dunkirk.

The Kriegsmarine lacked both ships, training and tactics to carry out large landing operations. It gathered in French and Belgian ports hundred of barges, but how these will behave in the Channel in autumn weather loaded with troops, tanks and so on was not assured. Also British bombers (the often overlooked part of the BoB) bombed regularly those ports and disabled a good part of the fleet.

As for an airborne invasion, paratroops won't be able to do much alone. The German paradrop on Crete was a very bloody affair, while the Anglo-Greek defenders were certainly weaker and with a worst equipement that what the British might have thrown against an invasion in 1940. And Crete had only some tens of aircraft to defend itself while Britain had hundreds

Any invasion would probably have been met with the full power of the Royal Navy. It is a fact that British destroyers had retreated from the Channel at the end of July 1940, but they could have been based in ports outside Bf 109 range and still raid the Channel at night or in bad weather days. And the biggers guns could have come too. At the time torpedo bombers were not operational on the German side, and I doubt the Luftwaffe would have been able to stop the Home Fleet going to the Channel. Probably it will have suffered, especially from Stukas, but far less than the German flotilla once it had reached it.