Basically, what I'm asking is, what would need to happen, what sort of misinformation would have to fall into German hands to make the Nazis think that it'd be a good idea?
And mind you that this is an idea so stupid that even Hitler got cold feet.
Move all of Fighter Commands serviceable aircraft to air fields further to the north, from RAF Wittering and beyond, and get them to play dead for a couple of days. That should convince the Luftwaffe that they have cleared the skies. It's a gamble and should only be attempted if there is confidence that the RAF can re-establish operations in SE England at a moments notice though.
If you look at the "Glossary of Sea Lion Threads" pinned near the top of the forum you will find quite a lot of Sea Lion FAILED threads. In many of these the OP is besides himself with joy at what a incredibly original idea he has come up with.