No, all that would be needed was air superiority over the Channel and parts of southern England, plus an open sea corridor. At most, Germany would have been delayed for several months, perhaps a year before sufficient transport caty could be built to establish a beachhead. Once a decent German force had landed, there is no way they would have been stopped on the ground. 500k troops would have been sufficient to defeat the British forces on home soil, since a lot of heavy equipment had been lost at dunkirk in 1940 meaning German tanks would be almost impossible to stop.