I3BrigPvSk
The Viking
Deploying Uboats to the channel in 1940 would have been a really quick way of removing the Uboats threat, it wasn't until August 1942 that Germany could field 100 operational U-boats do you really think it would be a good idea to stick the ones they had in a relatively shallow enclosed space to take on the home fleet?
As a quick comparison:
British and French ships (home fleet) 1940 ~200 (does not include fleets outside British and North Atlantic waters)
German ships and submarines (total) ~90 ( this includes pocket battleships at sea in the Atlantic)
The Uboats don't need to enter the English Channel to create a blockade and make it difficult for the RN to enter the English Channel. If the Luftwaffe had been successful with defeating the RAF, the German invasion force had probably wanted to cross the English Channel at Dover-Calais. The Germans could have used mines as well to disrupting the RN's movements.