F.6/42 was an Air Ministry/RAF RFP for interceptor fighters powered by 2000-2500hp class engines (Rolls-Royce Griffon, Napier Sabre and Bristol Centaurus). These aircraft were expected to be fast, fast climbing, very maneuverable (especially given their engine power and size), and armed with 4x20mm cannons. The ultimate winners were the Centaurus and Sabre powered versions of the Hawker Fury (though not actually part of the original tender, though was submitted as a private venture to the RAF as a successor to the Hawker Typhoon/Tempest), and the Folland Fo.117. The Fo.117 was never actually built and the Centaurus Fury was never ordered by the RAF in quantity (though it would form the basis of the RN's Sea Fury), and the Sabre Fury was ultimately cancelled in favor of jet fighters.
However, I'd argue that the Griffon Spitfires, namely the Mk 14 and Mks 21, 22 and 24 made the F.6/42 aircraft obsolete to requirements even before any of them flew, mostly by meeting most of the requirements out of the gate (though not designed for them explicitly), and getting into production sooner.
As mentioned, the Fo.117 was never built or flown, and the Hawker Fury didn't fly until the Sept. 1944. By then, the Spitfire 14 has been in service for several months. It would seem that the Spitfire 14 met or came close to meeting the F.6/42 requirements in most areas. The only areas that the 14 lacked in was maybe top speed (though marginally) and the fact that it didn't carry 4 20mm cannons are standard armament (the C and E wings were capable of it, but usually carried either 2x20mm/4x.303s or 2x20mm/2x.50s).
Of course, the Spitfire 21/22/24 it could be argued met that spec even more closely, though again weren't developed for it (basically intending to bridge the gap between the Spitfire 14 and the anticipated Spiteful with its strengthened wing and improved ailerons). Of course, jets like the Meteor and Vampire basically killed the Spiteful, but the 20 series Spitfires also (IMO at least) didn't help. Though slower in a straight-line than the Spiteful, the 20 series Spitfires had the same climbing ability, same 20mm cannon armament (as specified in F.6/42, and with more RPG than the Spiteful and easier reloading/servicing), and handled better, especially on take off and landing (still had the disadvantage of the narrow main landing gear, but stall and low speed qualities were significantly better than the Spiteful).
I'd argue, in addition to emerging jet fighters like the Gloster Meteor and especially the DH Vampire (a single seat, single engine jet answer to the F.6/42 concept), the Spitfire 14 and 20 series were sufficient to meet the F.6/42 requirement in all but name, and were in production much earlier than the never built Fo.117 and the ultimately prototype only RAF Fury, though that program did give the RN the Sea Fury.
Any thoughts on this having happened, even if it was only a coincidence?