To answer the question in simple terms - during the time period of the purposed operation Germany did not have the resources necessary to launch a successful sea and air invasion of the British Isles.
Now if your speaking in theory - a successful invasion would of been possible given time and resources. Niether which Nazi Germany had, given the desire to invade Russia during the same time period, and the intelligence that was being gather that showed that Russia was indeed rebuilding their military.
Futhermore when speaking of historical theory - If Germany would of concentrated on taking out the British Isles by sea and air borne invasion - would Russia of quietily sat on the side lines, or would Russia have attacked into the weaken eastern flank of the Germany? Knowing history and that Stalin wanted to control more terrority - I do not believe the Soviet Union would of waited for Germany to completely successful in Operation Sea Lion. I personally think that the USSR would of attacked into Poland once Germany was fully committed into invading England.
The amount of resources needed to launch an invasion that would have had a chance of success would of seriousily weaken Germany for a defensive operation in the East.
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