Glad we managed to address this question in the rules - yes for an amphib it is one infantry + one landing craft for your front line guys, of course your reserves can land (into reserve) without a landing craft (3 infantry or two of anything).
I get your point on the Sealion comment, but in the real war, the Germans did not plan on invading England until like July 1940. In your scenario, the Germans decide to go all out planning and building for Sealion in September 1939 - which probably would give them a lot more capability. But, if the Germans go all out to build like 4 more transports and 5 landing craft from the start, they will be investing more than 50PP instead of building infantry, armor, planes, subs and other priorities. It may make the taking of France more difficult. Also - if the British see all these naval biulds coming, they are probably not going to sit idle and will probably shift more of their strength and builds to deal with it. Only two British spaces are in range of fighters based in France (assuming the Germans take out France) making England not a push over - even with an all out German naval strategy.
That being said, I have lost England once to an aggressive (and surprise) Sea Lion in the Spring fo 1940 (Germany invaded England even before the Fall of France and I was not paying attention to the German builds and my defenses). . .