I've been running a blockade of England for around three months, running an average of 3-4+ transports sunk per day without any noticable effect on British forces. I was a bit curious, so I loaded a save as England and found to my suprise the British merchant marine was a little larger than expected. Neglecting any ships currently making trips, they had around 600 transports to spare. Poor Admiral Donitz is going to need a LOT of reinforcement.