From the early 50s the UK worked on a number of 20,000 ton light fleet carrier concepts, including variants designed to operate 28 Scimitar size fighters. The concepts included new build versions with very large sponsons for an almost parallel port deck and a starboard sponson, out board of the island, large enough to be used as a deck park. Another sketch that could have been used to upgrade the 1942 CVLs relied on a minimal angled deck and a switch back from concurrent launch and recovery evolutions to separate cycles to be conducted sequentially. While the size was similar to the 1942 CVLs (obviously for the modernised 1942s) the speed was to be higher at 28kts (re-engining for the modernised ships?). They were also designed with the possibility of converting them to VSTOL carriers once the technology was perfected and suitable aircraft became available.
There were also larger concepts, up to 30,000 tons, that fell between the modernised Colossus and Majestic class carriers and the reconstructed Victorious that were not so attractive as they were too large to be affordable light carriers and too small to be effective fleet carriers. This also led to other sketches for 35,000 ton ships of Victorious / Hermes size, it was these that due to scope creep, evolved into the 53,000 (maybe 54-55,000) ton CVA-01 designs.
My supposition is that when the CVA-01 was cancelled and the Escort Cruiser (intended class of five to serve as carrier escorts and independent ASW task force leaders) then the Command Cruiser (Invincible became the primary focus of government to provide the RN with the required command and ASW helicopter (even at the early stages also BEARCAP V/STOL fighter) cover, they instead revisited the 20-25,000 ton Light Fleet and 30-35,000 ton Fleet Carrier options that many design studies existed for. This would have made sense as many of these concepts were only 10 or less years old, they had been specifically designed to operate larger and heavier aircraft (i.e. deck strength, hanger height, deck park arrangement and size, catapult and arrester gear rating).
Also at this time the RN had conducted a number of studies, such as the one comparing the effectiveness of an all Skyhawk air group on Hermes instead of Sea Vixen and Buccaneer, but also one comparing an evolved P.1127 (the Harrier) to what became the Jaguar. The results were very interesting with it being determined that eight Jaguars could do everything twelve Harriers could (as well as things the Harrier couldn't), while being considerable cheaper, so much cheaper in fact that the a new generation CTOL Light Fleet Carrier would actually be more affordable than the same number of smaller Through Deck Cruisers with Harriers.
There is also the fact that a modern light fleet carrier could have been designed with sufficient steam to maintain over thirty knots while charging the catapult, apparently the limiting factor on Hermes operating Spey Phantoms. Even excluding the Phantom the CTOL configuration would have permitted the operation of fixed wing AEW and possibly ASW aircraft, considerable increasing the effectiveness of the fleet.
My scenario is, faced with the data provided by the RN, who (instead of assuming that the utility, flexibility and necessity of a balanced fleet built around carriers was self evident) managed to explain the need for carriers and there cost effectiveness compared to any other options, the British government agreed to a program of smaller fleet carriers to replace the existing ships. Further more, with the cancellation of Harrier in favour of the cheaper AFVG/UKGV and SEPECAT Jaguar, there was more money freed up for extra hulls meaning carrier number could be maintained at five, the way this was sold to the government was five 30-35,000 ton Fleet carriers (150-175,000 ton total) provided more capability than either six 18-20,000ton escort / command cruisers (108-120,000tons) or three 53-55,000ton Strike Carriers (159-165,000tons total). The advent of this British program also provided an affordable, modern design that attracted the interest of Australia, Canada and the Netherlands to replace their existing ships.
The first ship was ordered in 1967, laid down in 1969 and commissions in 1975 with subsequent ships laid down (and completed) at three year intervals. Their entry into service was covered by retaining Hermes in her CTOL configuration, as well as repairing and retaining Victorious, Centaur being converted into a Commando Carrier instead. The first ship replaced Centaur as a CTOL carrier in 1975, Ark Royal retiring in 1978, Victorious in 1981, Hermes converting to a Commando Carrier in 1984, and Eagle retiring in 1987. The air group of the first ship, HMS Queen Elizabeth, on entry into service consisted of 12 Phantoms, 12 Buccaneers, 12 Skyhawks, 5 Gannet AEW and 2 Wessex SAR, as did that of the second ship, HMS Prince of Wales. The third ship, HMS Duke of Edinburgh, introduced Tornado FAW1 and SR1, as well as Jaguar FRS1.
An interesting side effect of this change was the increase in the number of Commando Carriers from two to three permitting a more sustainable maintenance cycle and intern resulting in an improved material condition of the ships and a longer service life. Also the Type 82 program was expanded to eight ships, one fore each Fleet Carrier and Commando Carrier, while the eight County Class DLGs, were progressively converted into DDHs with Sea Slug being removed and extensive helicopter facilities being installed aft, together with Sea wolf being added with one launcher in B position and a second on the roof of the much larger hanger (four Seakings). The Type 22 Frigate and Type 42 Destroyer proceeded but with sixteen of each being ordered to provide each group with a pair of each, the major different from reality is they were configured to operate a single Seaking and a single Lynx each. Eight additional Type 21s were built to a modified design and they provided excellent service as GP / patrol frigates, as well as being successfully exported to Australia. Finally sixteen Type 23 Frigates were built to replace the remaining Leanders.
Following the completion of the last Fleet Carrier the first of three modified versions was ordered as a replacement for the aging Bulwark, entering service in 1990, 93 and 96. Then came the new DDHs, these were a return to the late 60s Escort Cruiser configuration, displaced 20,000tons and resembled a more cruiser like Invincible. Somewhat ironically, when the UK cancelled development of the Harrier the USMC funded its resurrection as they believed the capability to be critical. The resulting aircraft was a hybrid sharing features of the Sea Harrier and the AV-8A, with all being radar equipped, though they were very much attack aircraft with only a secondary fighter role. To shore up UK production rights the UK government ordered 100 aircraft with the intention of it entering service with the RAF, however the types suitability for operation from the Commando Carriers was without a doubt, leading to three squadrons being assigned to the fledgling Royal Marines Aviation Group. An evolved version incorporating many systems and components of the McDonald Douglas AV-8B (more powerful engine, composite structure etc) led to the development and deployment of the Sea Harrier F/A-2, not only with the Royal marines for operation from the Commando Carriers, but also with the RN to operate of the new Escort Cruisers.
