Jack,
Although we won the Battles of the Atlantic in both WWI and WWII, it still true that a submarine blockade is the means by which this country can be brought to its knees in a future war.
It was our good fortune that both the Kaiser and Hitler were land animals, neither appreciated that, if they had only put sufficient resources into their U-boat campaigns, they would have had a good chance of cutting the lifeline which supplied us with food, raw materials and war supplies. As we cannot ourselves produce the food the country needs, all an enemy needs to do then is to wait until we are starved into submission.
Even as it was, Dönitz wasn't doing too badly. Churchill said that this was always his worst worry of the war. Admittedly "Ultra" information was delayed by the time needed by the cryptographers and translators of Bletchley Park, but there were times when Churchill had on his desk a transcript of Hitler's orders to some General in the field before the recipient himself had seen them.
This was particularly useful in the U-boat war: it enabled the Admiralty to re-route convoys away from the "Wolf Packs"; and on occasion, when a U-boat "mother ship" had been located, it was arranged for a recce Catalina to "spot" it (purely by "chance", of course) so that the subsequent fatal attack would be obviously the result of the "sighting". The "chicks", deprived of fuel, stores and torpedo replacements, had no choice but to cut their sorties short and return to port.
Danny.