AU-boat that was headed from the open sea towards a safe port on Fortress Europe would just go to the French ports on the Bay of Biscay—Lorient, most likely. Getting to Germany’s North Sea and Baltic ports would be a far longer and more complicated and dangerous trip. The U-boat would have to get around Great Britain somehow. To the south, it would have to make a dash up the Channel, which (setting aside that it’s a bottleneck, crackling with British radar) has been turned into a maze of sunken block-ships and minefields by those Royal Navy spoil-sports. There is a lot more room up north.