The ‘T’ class 1940 designs were slow but of long endurance, although because of their limited size they were not capable of supporting their crews for such a length of time.

British submarine development was influenced by the cruiser and fleet submarine concepts. The main thrust of early evolution between the wars centered on the overseas patrol type, which displaced 1,475 tons on the surface and had a range of 10,900 miles at 8 knots, a submerged endurance of 36 hours at 2 knots, and a diving depth of 500 feet. Armament included a battery of 8 torpedo tubes with 14 torpedoes and a 4-inch deck gun. A group of similar-sized minelaying submarines also was built, as was a small series of very fast large submarines for work with the fleet, but both of these developments proved very expensive and of limited operational usefulness.

In the early 1930s, a fresh start was made with the Swordfish-class, which was designed for offensive patrols in narrow waters. These boats displaced 640 tons standard. They had a cruising range of 3,800 miles at 9 knots on the surface and 36 hours at 3 knots submerged, and they could dive to 300 feet. Armament was 6 torpedo tubes with 12 torpedoes and a 3-inch gun. A larger overseas patrol type, the Triton class, appeared in 1937. These displaced 1,090 tons standard; they had a cruising range of 4,500 miles at 11 knots on the surface and 55 hours at 3 knots submerged, and they could dive to 300 feet. Armament was 10 torpedo tubes with 16 torpedoes and a 4-inch gun. Britain concentrated its production of submarines during the war on these two types, producing a total of 62 of the S type and 53 of the T type.

Just before the war, the Royal Navy developed a small submarine for training not only crews and new commanding officers but also antisubmarine vessels. When war came, the design was quickly adapted for operational use, and the submarine proved particularly useful in confined waters such as the North Sea and Mediterranean. The U class displaced between 540 and 646 tons on the surface, with a range of 3,600 miles at 10 knots on the surface, a submerged endurance of 60 hours at 2 knots, and a diving depth of 200 feet. Armament included a battery of 6 torpedo tubes with 10 torpedoes and a 3-inch deck gun. A total of 71 boats were constructed of this class and its slightly improved successors of the V class. Although they were useful boats in the early part of the war, the later examples diverted resources from construction of more effective vessels. Britain also built some 36 midget submarines; with 4-man crews, these vessels attacked ships at anchor in harbor.