Forgive me as a mere history-graduate and aero-sexual but here are a few to my mind: First the almost-have-beens:
Saunders-Roe SR.177 - only conspiracy theory that it was scuppered for purchase by nefarious dealings by LM?
Phase 2 Vulcan - dealt a fatal blow by the death of Skybolt but still a formidable machine.
Lightning F6 with guns - lessons learned by now (or not)?
Now the wishful thinking:
Vought F-8 Crusader: Of the same vintage as the Scimitar and Sea Vixen but in a entirely different class
Vought A-7 Corsair II - A strike jet par excellence. Had bombing performance (thanks to CCIP) far in advance to the contemporary Phantom yet far less famed.
P.1216 Prototype - Who knows? It made it barely beyond the drawing board. But it looked good!