Since the topic is the better British submarine service for WW II, let one address that issue.
First, no outlandish extrapolations. The British have limited resources to assign. ASW is the highest naval priority, fleet readiness the second highest, fleet air arm next, and on down the line so in any ATL that is even plausible, the subs will come just ahead of the RN ice cream budget.
Given the low priority, the mission drives the need.
Two viable operational theaters. Pacific (lower priority) Mediterranean (higher priority).
British Med boats look to be the U and V classes.
Expect these boats to be slaughtered., The Italians are GOOD at ASW.
These boats desperately needed better induction valves, thicker pressure hulls, a working acoustic soda can decoy system, depth control arrangements, electric motor and screw silencing, a dived camouflage scheme, and better tender support services for when they came in damaged after a patrol.
Nothing wrong with the crews or their skippers. The British are one up on both the Germans and Americans there at the war's start.
The T-class and others like them appear tailored for the Pacific.
Gawdawful boats. They and the A class which followed them, seem to have been designed to kill their crews. Once again, silenced electric motors, better induction valves and simpler dive controls would have helped.
British construction methods and workmanship seems to have been a problem in any of their submarines,
so one critical area has to be quality control of manufacture.
The British were not alone in this last issue. American shipyards and GOAT ISLAND naval torpedo station in the 30s and early 40s killed several hundred US submarine sailors in WW II due to their incompetent workmanship and inadequate quality control.