Remove Sealion from your brain, and look at the question from the WW1 perspective. If it became strategically viable to invade Britain, and enough whittling down of the Grand Fleet had occurred to make it feasible, why would the Germans cross the Channel to do it? The fact that the Nazis viewed the invasion as a glorified river crossing does not mean that the Imperial German army will, and there are enough seaborne invasions in the early 20th century to show that converted barges are NOT a pre-requisite. Japan's invasion of the Philippines is a perfectly good example. Take transports under close escort across the North Sea, land in Suffolk somewhere, maybe even use the famous zepellin-behind-the-lines idea to get elite teams into the British rear, and then seize a port of some kind.
Best Regards
Grey Wolf