Okay, lets assume that Nazi Gemrnay goes ahead with Sealion in 1940/41 (perhaps before Barbarossa) and it fails spectacularly. The Royal Navy blows the German ships out of the water, those that reach land are easily mopped up by British troops and the Luftwaffe forces which took part are wiped out.
Lets say eight or nine divisions are lost, one of them a panzer division.
British casualties are light-ish. The Royal Navy loses a destroyer or two, maybe a cruiser and a handfull of smaller ships. The RAF takes the brunt of the losses, many planes are lost as are plenty of soldiers on the ground.
How does this affect the rest of the war? Will Stalin take advantage of this German failure?