.... So starting with a clean sheet of paper in 1950, and assuming the successful design should stay in service for at least a decade, what would a worthwhile RAF fighter look like? Feel free to specify engines, armament, systems, or anything else.
Let me propose a start date of 1951: Helping out in Korea, the RAF first encountered the Mig 15 and got a naughty shock as it realized how far ahead the Russians were, compared to their own frontine fighters of the time: The Gloster Meteor, the DH Vampire and the Supermarine Attacker. Even the Vampire's upgraded successor, the Venom, just now entering service would be seriously challenged.
Looking at the Mig 15 and comparable aircraft like the USAF's F86 Sarbre, the following specifications are drawn up:
=> one person air superiority fighter
=> swept wing or delta wing
=> sufficient speed and range to take off from the RAF's West-German bases and engage a Warshaw Pact fjghter/bomber force before it enters NATO airspace.
=> possibility to upgrade to a radar equipped all weather fighter
=> if the project is still in planning by 1955, add supersonic capacity.
=> all-cannon armament if planned before 1955, air-to-air rockets afterwards. This means that a "winning" design that will stay in service for ten years or more will be either upgradable to rockets, or replaced by a mid-life redesign mark II that has AAR systems built in.
OTL, 1955 saw the introduction of the Hawker Hunter and Hawker Seahawk, later the Blackburn Buckaneer, Blister Javelin and English Electric Lightning as well as the not quite that successful Supermarine Swift and Scimitar. As all those aircraft were already in planning or flying as prototypes, given the right priorities their entry into service could be pushed forward by two, may be three years.
Furthermore there are two experimental research aircraft that possibly could be developed into great fighters: The Fairy Delta I and the Avro 707. By chance both of them deltas.
Finally, there is the possibility of a cooperation with either France or West German like OTL would happen in the sixties. I know this soul be a tricky subject in 1951, but with the right emphasis on 'we are all NATO now', could be pulled off by 1955, (perhaps pushing the otherwise unsuccessful 1955 Mirage 1 before it got developed into the 1959 Mirage III)