The question describes the traditional picture of British anti-submarine efforts in the interwar period. It is ultimately derived from Stephen Roskill's Naval Policy Between the Wars, and to a lesser extent from Arthur Marder's monograph The Influence of History on Sea Power. These two authors were hugely influential on the historiography of the Royal Navy - Roskill wrote the Royal Navy's official history of the Second World War, while Marder wrote one of the most detailed works on its actions in the First. Roskill alleges that the RN believed that the development of ASDIC (sonar, in American parlance) made the submarine useless. He claims that 'not one exercise in the protection of a slow mercantile convoy against submarine or air attack took place between 1919 and 1939’, with the RN choosing instead to focus all its anti-submarine warfare (ASW) efforts on hunting submarines. Marder, meanwhile, argues that the interwar RN failed to adequately study the lessons of WWI. In Marder's picture of the topic, the Navy chose to focus its efforts more on the more spectacular areas, especially its failure to defeat the German fleet at Jutland, over ASW. He describes a navy that, in its 1934 Staff College course, spent three days on Jutland, and just one hour on anti-submarine precautions. Most later authors have repeated the assertions of these two authors, furthering the picture of the RN as rejecting the convoy system and other lessons of WWI. However, more recent scholarship has reassessed this picture. In particular, George Franklin's work has painted a completely new picture of the RN's thinking on ASW in the interwar period. This answer will reflect the newer scholarship.

In the 1920s, the main strategic threat to Britain was not Germany, which had been neutralised by the Versailles Treaty and had a nominally friendly government. Instead, the key threat was from Japan. While Britain and Japan had been allies, differences in foreign policy, especially over the 1923 Washington Treaty, forced a wedge between them. As a result, the Admiralty's planning for a war in the 1920s focused primarily on Japan. While British trade routes would be hugely vulnerable in the event of such a war, the Admiralty appears to have correctly assessed Japanese doctrine. The Imperial Japanese Navy put little emphasis on the submarine for hunting merchants, choosing instead to focus on raiding cruisers. The IJN's submarine force would, instead, hunt for the enemy's battlefleet. As a result, British planning for commerce defence in the 1920s focused mainly on the surface threat, moving away from convoy to some extent. From 1932-3, though, it became clear that Germany might not be as quiescent as was earlier thought. It soon became clear that Germany intended to build up a submarine force, a fact made abundantly clear by the 1935 Anglo-German Naval Agreement. A major review of ASW efforts was carried out by the RN in 1932, with Germany seen as the main threat, despite them not having any submarines available at the time. In 1936, the tri-service Joint Planning Committee estimated that, by 1939, the German Navy would have between 56-66 submarines in operation (the real number, for September 1939, was 56). To this end, the RN began to refocus its efforts on protecting commerce against submarine attack. Convoy was generally understood to be part of this protection, especially if Germany resumed its policy of unrestricted submarine warfare.

The RN's main anti-submarine establishment was the Anti-Submarine School at HMS Osprey, Portland. Osprey had several main roles; as well as training officers, ASDIC operators and ships' crews, it was also intended to develop new tactics and equipment for hunting submarines. Osprey trained specialist ASW officers for the staffs of fleets and destroyer flotillas. It also hosted a 15-day course for officers on ships without one of these specialist officers. Every ASDIC operator in the fleet was trained there, and all destroyers and escorts spent time at Osprey when they were working up, to gain experience in ASW. At the School, training would include time spent in shore-based simulators, and exercises at sea. Many of these exercises would involve the ship protecting merchant ships, as they would in a convoy situation. While single ships were often used, these were generally understood to represent a full convoy, as providing such would be too expensive. Between 1927 and 1938, records describe nine large-scale convoy exercises, involving between four and 18 destroyers as escorts, and up to at least ten submarines in the attacking force. Five of these, in contradiction of Roskill's statement, were slow, with speeds of below 9 knots. These large-scale exercises appear to have been accompanied by additional, smaller-scale exercises, of which details have not survived. This meant that there was a general understanding of good convoy tactics. However, this was accompanied by a belief that convoy had to be accompanied by hunting as a way to kill submarines. While this would prove to be somewhat true, the convoys had to be adequately protected first. Due to a lack of submarines, there were few opportunities to test 'wolf-pack' tactics, with multiple submarines attacking. Similarly, policies restricted the ability of submarines to act on the surface at night. While there was some understanding of the threat these tactics posed, the true effectiveness was unclear, meaning that the protection initially given to convoys would prove insufficient.

The first RN guide to convoy tactics and formation in the interwar period held the ungainly title of Mercantile Convoys: General Instructions for Port Convoy Officers, Ocean and Destroyer Escorts and Commodores of Convoy. Issued in 1919, it was intended to condense down every lesson learned during WWI. In 1934, it would be updated and streamlined into the Mercantile Convoy Instructions. The year after this was published, Paymaster Rear-Admiral G.H.W.Eldon Manisty was appointed to the Naval Staff. His role was to set up a Naval Control of Shipping organisation (NCS). The NCS would provide officers to guide and form convoys in the event of wartime. Manisty toured the world, appointing reservists for this organisation in likely assembly points. Two years later, Admiral W. James formed the Shipping Defence Advisory Committee, with Manisty as his vice-chairman. This brought together naval officers, ship owners, Ministerial representatives and other technical experts to hash out methods of commerce defence. Convoy was emphasised here, with James stating that 'the convoy system is considered by the Admiralty to be the most effective form of protection against surface, submarine or air attack'. This was not an isolated opinion. Captain T.S.V. Phillips, the RN's Director of Plans, would write in a 1938 memo that

… it is open to doubt whether the delays due to convoy will be any greater than those caused by evasive routeing and shipping being afraid to sail on account of real or imagined dangers. … Moreover, if, as seems probable, losses in convoy are considerably less than losses in independent sailings, then the number of ships available to carry cargoes will remain greater under a convoy system.

Phillips' memo sought to shorten the wait between the start of any conflict and the implementation of the convoy system, through methods like the NCS and increased mobilisation of the Reserve Fleet. His arguments, and those produced by the reports of the SDAC, were taken up by the highest levels of the Admiralty. The result was a new 1939 edition of the Manual of Anti-Submarine Warfare, which emphasised the utility of convoy in protecting merchant shipping. However, British planning was also flawed. Subs travelling from German bases would have to make a long journey around Scotland to reach the Atlantic convoy routes; they would likely not have the endurance to range deep into the Atlantic after such a journey. Given this, it was assumed that the submarine threat would be greatest in waters relatively near to the UK, especially in the heavily trafficked waters of the East Coast and Western Approaches. As such, British building programs focused on coastal escorts, like the 'Hunt' class destroyer escorts, Kingfisher class sloops and 'Flower' class corvettes. These ships would prove to be insufficient after the fall of Norway and France gave Germany bases closer to the mid-Atlantic.