In his book, "Invasion, the Alternate History of the German Invasion of England July 1940", Macksey has the Germans deviate from our TL by setting aside five divisions in early June immediately after the BEF is evacuated but whilst the French are still in the game South of the Somme.
The Germans manage to control Dover straits for a critical period, and manage to pull it off before the Brits really get their act together after Dunkirk.
I actually found a review by our well-respected member Amerigo Vespucci on the Amazon site and hope he doesn't mind me quoting it in full:
"This was the first Alternate History book that I read, and to me, it has always been the best. The story details a successful invasion of Great Britain in 1940, from the planning stages through the air battles, the glider landings, the beach crossings, and the eventual surrender of London.
The text is helped enormously by several detailed maps that enable the reader to completely understand the author's view of what he believes would happen. I found the story completely plausible, well-written, and I agree with the conclusions reached by the author. A previous reviewer wrote that it seemed to him as if the author portrayed the British Army in England as not being able to defend against a single German soldier with a rusty spear. The fact of the matter is that that isn't far from the truth.
The Royal Navy in the book comes through as rather shallow, but I would not disagree much with the author's conclusions. The Royal Navy could not stop an Operation Sealion with the entire force of Kriegsmarine and Luftwaffe facing it. It could only hope to slow it enough to help the British Army destroy the landings, a possibility slim at best. I agree that a German invasion would succeed, albeit at slightly higher casualties in both men and naval forces than the author has portrayed, but I cannot disagree with his conclusion. I had no problems with the book, excepting one. I wanted more! "