I hate to interrupt a fascinating discussion of army personnel policy, but frankly, I think you're busying yourselves with secondary details.
There really are two central questions that have to be addressed in any Sea Lion discussion for an ATL, and if you can't answer these decisively, it cannot possibly matter whether the ratio of British defenders it takes to stop the German attackers is 1:1, or 5:1, or 20:1, or 50:1.
1.) What will you do to cross the channel and avoid the Royal Navy, which, unlike the army, is actually Britain's main line of defense?
2.) How will you prevent the British from easily countering whatever it is you propose to do to solve question #1?
The British have substantially more ships than the Germans and also substantially more shipbuilding capacity. "Germany should just start building landing ships," or now rocket missile ships (WTF????) apparently, presumes that the British will just observe these developments and do nothing. Why on earth would the world's largest navy allow unchallenged a potential rival to build a force capable of invading its homeland? This is absurdity. You can't just "go backwards" a few years to start the planning and assume that the British will spend those years doing nothing in response to an obviously mounting threat.
Previous threads have repeatedly demonstrated that the Sea Lion plans called, as they would have had to, for the German forces to begin assembling days prior to the actual landings. I seriously doubt there is a single historical precedent for a successful amphibious operation against a major power in the face of enemy naval superiority by said power and without the element of surprise, but perhaps someone can dredge up an example from God knows where. The almost certain reality is that upon realizing an invasion is genuinely underway the British will flood the channel with every armed ship they can find. This will result in serious losses from the Luftwaffe, but I'm guessing a few hundred military vessels can sink some barges faster than they can be sunk themselves by an air force that is still very new to the anti-shipping game.
In those threads, people speculated a number of potential ways to make the operation feasible. For instance, they have speculated that instead of invading England proper the Germans should have headed to the Isle of Wight. They have also suggested that the Germans mount anti-aircraft guns on the decks of their barges so that the barges can go toe-to-toe with Tribal-class destroyers. Want to go down that route again?
I don't want to sound too derisive here. People who ask "the Sea Lion question" have their heads in the right place for alternative history, usually anyways, but I just don't think this is the right place to spend their energy. Surely there are other important turning points in history that were much closer-run things and therefore much easier to push back and forth from an AH perspective.