I saw a very interesting programme about the invasion of Britain from the British military side.

Sealion wouldn't have worked but if the Germans had launched a airborne assault during or straight after Dunkirk to capture British airbases in the south of England and then flew troops in, there was very little they could of done about this.

Morale was so low at the time in Britain and Churchill (don't forget) was not liked or trusted by the establishment. Its quite possible at this time Britain would have sued for peace.