... basically, I'm looking to outfit the RAF with something not worse and not later than Mirage III, Drakken, MiG-21 or F-8 Crusader.
Why 1-engine? To help sell it abroad, of course, thus the UK government can also buy plenty of them, should also help the industry because of a more numerous production numbers.
Extra points if it can be easily navalized for FAA 'classic' carriers (= no STOL).