Lets speculate that
Operation Sealion was a tremendous success. British forces are being pushed back towards London from the south of England. Does the US step in to assist our close friend and ally? Does the US wait and build up forces in another location? What are the possible launching points for a US/allied resistance re-conquest of the U.K./Europe? Would Ireland waive its' neutrality and allow the US a place to stage its' military? Or would Spain be an alternative starting point? Say the British Army is hopelessly scattered. Could the Canadians and US Armies alone defeat the Germans or at least beat them back enough to re-organize the British Army?