After reading and commenting on the recent post about the possibility of a German invasion of Britain (here: http://www.reddit.com/r/AskHistorians/comments/25cri9/if_hitler_was_willing_to_sacrifice_a_ton_of/) I am curious: how well defended was the Channel at any point in time? Commenters on the other thread mentioned that during a war game simulation of the invasion, the British required 24 hrs to fully assemble their destroyer fleet for a counterattack on the invasion force. How well defended was the Channel? Were there mines or SONAR nets? Artillery emplacements? Did U-Boat squadrons dare to operate within the Channel or was it too dangerous? I'm interested in any information about the defenses at any stage of the war, so that I can get a picture of how the defenses were built up over time (as I presume they were).
TL;DR What were the English Channel defenses like during WWII, when the Germans were a stone's throw away from Britain?
My favorite Channel defense was the tiny Romney Hythe & Dymchurch light railway coastal armored defense train. The efficacy of a 15 inch gauge armored train is highly questionable. To me, though, it epitomizes the idea of 'British pluck.' If it could be used to resist the Germans, it was bloody well going to be employed to resist the Germans.
I can't speak to the ground defenses past the employment of this train. The British Army was in the process of rebuilding their force after Dunkirk, where they lost a good deal of equipment. However, the idea of resourcefulness in defense extended to air defense. Civilians helped with spotting duties, fire brigades, and rounding up downed German pilots. This was in addition to the radar net, military spotters, military units dedicated to listening for German planes with specialized equipment, the airfields with their air crews and ground crews, and anti-aircraft artillery units. Further, the Royal Navy was fully up to speed and ready to respond.
In my opinion, given the RAF's survival and the continued existence of the Royal Navy, even the tiny armored train mentioned above was more than sufficient to guard the Channel. Without knocking out the RAF and neutralizing the Royal Navy, any landing would have been extremely ill conceived.
