An amphibious invasion only requires the sea zone it is being launched from to be cleared of enemy units.
If the Brits leave the English Channel empty, Operation Sealion is one step closer to success. It does not matter if every other sea zone on the map has enough allied ships to fill the ocean, all that matters is the English Channel has no defending units.
For this discussion, submerged submarines do not count as opposing units. Fighters do, even if their CV is sunk.