The Germans build more subs, the Brits delay the introduction of the convoy system longer. IIRC the latter was done at the insistance of the French and it lead to a quick and big drop in sinkings.
In this case WW2-like wolfpacks are not needed. Frankly with ships all over the seas subs operating alone is a better tactic. Geographically it would be more like a "Battle of the British Isles" because there are no airplanes that have the range to chase the subs deep into the Atlantic.