At the start of WWII Churchill sent out the Royal Navy's Fleet Carriers as part of Uboat Hunter Killer Groups. He had the right idea as events later proved but they were the wrong ships for the job. These Uboat hunts achieved nothing except the loss of HMS Courageous and the near loss of other carriers and had to be abandoned. What if there had been another option to the fleet carriers though? What if the Royal Navy had the commerce protection carriers it had always wanted. They couldn't be mini fleet carriers as that would have clearly broken the Washington and London naval treaty limits. What if though they were part of the Royal Fleet Auxiliary? Large supply ships of around 10-12,000 tons with side mounted funnels and pilot houses and a raised large flat platform built over the over the holds to allow aircraft to be carried on deck in addition to normal stores in the holds. As supply ships in peacetime they're exempt from the treaties but with war coming and the treaties expired could be rapidly converted to at least the standard of the later Merchant Aircraft Carriers and with a little more time into Escort Carriers.
Could the RN justify building say 6 supply ships/aircraft ferries in the 30's?
There are numerous problems.
The Royal Navy was an arm of the British government. These governments spent the period 1922 to 1936 trying to reduce defense expenditures through limitations treaties. The RN spent a godawful number of man-hours designing undersized BBs, carriers and cruisers to propose as examples of the qualitative limits the British wanted at the various Treaty conferences. The British also paid the most attention to the Treaties, using the least number of loopholes and legal fictions to get around the limitations. About the only trick they used was to design their ships to a standard displacement with a limited number of main armament rounds (60 per gun for battleships), while providing room in the magazines for 100, despite the Treaty definition of standard displacement as the "ship ready for war minus fuel and reserve feed water". Building convertible auxiliaries and merchant ships would be too much for the governments in power at the time.
The RN did not control the development and production of its aircraft and the provision and training of its aircrew. This resulted in lower priorities placed on naval requirements. Once it reclaimed control of the RNAS, it still had to operate within the limitations of British defense policy and doctrine. With the priority on the defense of Britain and bombers, it would take time for the RNAS to build the training platforms, cadre and aircraft to produce the additional aircrew, pilots and maintenance crews and to produce the necessary aircraft, even if purchased in the USA. Between the lack of slack for converting the dual-use auxiliaries and getting aircrews and planes, it could be the end of 1940 before any of these ships is ready.
RN doctrine and strategic planning between 1922 and 1935 was focused on the campaign against Japan in the Pacific. Ship design and organizational and training planning centered on collecting the Fleet in the Med and proceeding to the Far East, where the Fleet would operate from Singapore, seeking a decisive battle with the Japanese battle line and after victory, obtaining additional ports from the Chinese to use along with Hong Kong to blockade Japan and force her to sue for peace. Between 1935 and 1937, it shifted to a war with Italy and then from 1938 to a war with Germany or Germany, Italy and/or Japan. Only in 1938 was defense of trade an issue within the strategic and operational planning. Before then, the focus was on the Fleet, the Battle Line and its supporting elements, including the carriers. The RN, like the USN and IJN, saw the best defense against an enemies' carriers was pre-emptive strikes. But where the USN and IJN looked to fighters, even pre-radar, as an integral part of the fleet's air defenses, the British became convinced by 1936, that the bomber would always get through. So their response was an armored hanger carrier, where when enemy air strikes threatened, the strike aircraft would be struck below in the hangers and the defense of the Fleet left to its AA guns. On after drone targets became available, did the RN begin to question the viability of that doctrinal decisions and then radar made it the right decision in 1935 and the wrong one for 1940, since the armored carrier couldn't carry enough planes to execute strikes and defend the Fleet. Also, the RN never had much of an auxiliary force, something that would come to haunt it when it moved to the Pacific for operations. With the network of bases and ports throughout the world that the Empire afforded, the British didn't need and the RN couldn't justify spending limited resources on auxiliaries it couldn't really use in peacetime.
As pointed out, while the Treasury was not the drag on re-armament that is commonly thought, Britain's declining economic position and the atrophy in her ship building industry and supporting industries was. Britain simply didn't have the capability between 1938-39, when the London Treaty of 1936 collapsed and when the war started in 1939, to do everything it needed to rebuild its air force, navy and army. Even BBs had to wait for dock or pier space and for armaments production to accelerate to get modernized. The RN desperately wanted more cruisers, to modernize HMS Hood, to get more destroyers and the war came three years to early for that. It was a miracle for the British that the US had a President with the foresight to prepare the US for mobilization between 1939-41 and who was both navy and air minded and could push the escort carrier concept on the USN and get it into serial production to provide the escort carriers for the Battle of the Atlantic.