The invasion of 1940 was not ever likely to happen. Even if Germany had achieved air superiority by winning the Battle of Britain the Royal Navy would have stopped what little amphibious capability Germany had, evenb if it meant great naval sacrifices under German air power.

The worst of it for the UK was during the Battle of the Atlantic before the U boat threat had been bested. Britain may have starved, and US involvement in WW2 initially did little to improve the situation. US shipping was already involved prior to USA being at war with Germany, and when war did come, anti submarine warfare was not especially an American priority.