By the time 1960's roll around, "upgrading" Hunter for the UK (or any other customer, really) is only meaningful for ground attack as was done IRL, or unless something is preventing the nation from acquiring modern fighters (but then again, where would the upgrades come from?). If it's internal politics that's preventing the purchase, well, upgrading Hunter with a radar nose, entirely new avionics, afterburner, strenghtening the wings, etc. will soon end up costing more than a new fighter would even if psychologically it would be easier to accept upgrades instead of new purchases (even the Swiss who had this problem did not upgrade them for air-to-air).
Before someone brings up the Vietnam War, MiG-17 was viable only because of unrealistic rules of engagement and faulty tactics on the US side, and contrary to the belief, most planes were not shot down after a dogfight but were surprised and never knew they were being attacked until they started getting missile/gun hits. After the US returned to the North in 1972 with relaxed RoE and better training, it was pretty much proven that subsonics don't stand much chance against supersonics because the latter can dictate when and how combat is engaged (ie. only when they're holding the advantage).
Beyond visual range missile truck is not very realistic either if it has to deal with enemy fighters, which is why the US Navy abandoned the F6D Missileer concept already in the 1950's (and subsonic interceptor in the age of supersonic bombers makes the concept even less realistic).