The RN didn't "approve" of submarines, and as a consequence RN submarines were not very good. Fortunately, RN sailors were very good and made boats such as the U Class do well in the confined waters of the Mediterranean. Here are some stats comparing the U Class to its German and US contemporaries:
U Class: 630//730 tons; 6 tubes, 2 external, bow only; 10 torpedoes; 11.25kn/10kn
Type VIIC U-Boat: 769/871 tons; 4/1 tubes; 14 torpedoes; 17.7kn/7.6kn; 750 feet max
Type IXC U-Boat: 1120/1232 tons; 4/2 tubes; 22 torpedoes; 18.2kn/7.7kn; 750 feet max
Gato Class: 1524/2424 tons; 6/4 tubes; 24 torpedoes; 21kn/9kn; 300 feet test
Balao class: 1526/2424 tones; 6/4 tubes; 24 torpedoes; 20.25kn/8.75kn; 400 feet test
One can infer differing roles for the several designs. The German Type IX U-Boats were intended to range the entire Atlantic Ocean, to the US East Coast, the Caribbean, and South Atlantic. USN Gatos and Balaos were designed with the Pacific Ocean and potential war with Japan in mind. German Type VII U-Boats were designed with the North Atlantic and war with Britain in mind. The U Class was designed to operate near the home isles and the Med.
Crew sizes varied by class, but I toured the Balao class USS Pampanito (used in "Down Periscope") and at 6 feet tall, it was very cramped. Scale that to a VIIC or U Class! USN submariners tended to be around 5'6", but still ... not for the claustrophobic!