Operation Sealion - intervention of Japanese or Italian navies.

Well, having the RM waiting in the Bay of Biscay might make Hitler to choose not to focus in terror bombing, so air superiority, at least in the channel, might be doable.
Naval superiority, even with the RM, it's another matter altogether. Even if the Germans can get across, they'll still need ships to escort resupply convoys. And the RN has more, and better, warships than the axis.
 
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