Von Manstein wrote about this in his book Lost Victories. Given that von Manstein was the best general of WWII, on either side, I take his opinion on these matters very seriously.
Von Manstein pointed out that the farther away a plane fights from its own airbase, the more of a disadvantage it has. This worked very much against Germany during the Battle of Britain. German fighters would use most of their available fuel flying to and from targets in Britain. They’d only have a small amount of fuel available for dog fighting. Whereas, British fighter planes fought very close to their own air bases. They could use most of their fuel for dog fighting, because very little of it was required to get to or from the place of conflict. Therefore, the RAF was able to exert a much stronger air presence than mere numbers of planes might suggest.
Von Manstein’s proposed solution to all this was to not bother with fights deep inside Britain. Instead, German aircraft would stay relatively close to their own airbases. On the other side of the equation, preparations for a land invasion of Britain would proceed very quickly. He had no objection whatever to invading Britain even before the completion of land operations against France. (As an aside: had von Manstein’s plan been followed in its entirety, the Dunkirk evacuation would never have happened.)
Von Manstein pointed out that Britain had a negligible defensive army defending its own island during the summer of 1940. (Though that army would have been even more negligible, had the Dunkirk operation been avoided.)
Once the Luftwaffe got airbases set up near the English Channel, then–at least for air battles over the English Channel–the range factor would not particularly have favored either the RAF or Luftwaffe. The Luftwaffe could therefore have exerted a much stronger air presence over the English Channel than the RAF, due to sheer numbers. The strength of the Luftwaffe air presence would have it impossible for the British to defend the English channel with surface naval units. The Japanese attacks on British battleships at Singapore, and the later American attacks on Japanese battleships such as the Yamato, demonstrate what happens when one side controls the air and the other controls the surface of the water.
With the Germans controlling the sky above the English Channel, and with them using that control to sink any British warship wandering too close to the planned invasion route, most of the German transports would have gotten through. Once German troops landed in Britain, they would soon have discovered that the British Army was in no position to resist a serious land invasion.
Von Manstein’s plan did, however, contain an element of risk. The plan Hitler chose involved pitting the Luftwaffe against the RAF over a period of months, to see which one was stronger. Only after the Luftwaffe had won this contest would Hitler go forward with a land invasion. The heart of von Manstein’s plan involved postponing the contest of aerial strength until the invasion itself, confident that the Luftwaffe would win. The contest of aerial strength von Manstein envisioned would have taken place on terms much more favorable to the Luftwaffe, than the contest Hitler ultimately chose.
While von Manstein admitted his plan contained an element of tactical risk, he stated that the strategic risk of allowing Britain to continue to participate in the war was much, much greater. He was right. In 1940, Britain produced more military aircraft than did Germany. In addition, Britain received an enormous amount of American military aid. British politicians had no interest in negotiated peace, so the war against Britain needed to be won in 1940 or not at all.
