Derek Robinson summed it up pretty well. The main transport for infantry would have been river barges, not exactly suited to open sea. The time taken to cross the Channel would have been 24 hours or more. All the Royal Navy had to do to win was send destroyers sailing among the invasion fleet at speed to sink most of these barges. This could have been largely done largely at night, negating the effectiveness of the Luftwaffe.
My father was in the RAF (ground crew) and I was and am very proud of him. However, if the RAF had been largely defeated, it seems the UK would have had another string to its bow.