The UK guns saga should be seen against the backdrop of the roles envisaged for the aircraft.
The Hunter (aah Hawker) was DFGA, thus guns were considered as dual role. The Lightning (WIWOL) was an AWF – actually all-weather interceptor, where the bomber opposition had guns pointing back at you - thus greater range, and radar ranging and direction. Missiles were designed to overcome these difficulties, so too the problems of much higher airspeeds. Guns made more sense in Europe (F2A) to engage low level strike aircraft.
There was (is) a requirement for guns in ground attack aircraft, unfortunately some were called ‘fighters’ which confused those who should have known better; however with the advent of modern ground-defence systems the debate for guns is open.
The F4 gun was for soft air-air targets in VN; the UK eventually acknowledged the F4s dual role in UKADE and Europe.
The A10 fills a niche role against poorly defended ground targets or had sufficient ECM cover to make it viable.
The future balance might be with a low cost gun attack (depending on the capabilities of ECM suppression) vs high cost stand-off precision weapons.
Yet as we should remember we never fight the war for which we train for (or plan for). No plan survives contact with the enemy – i.e. current sand-wars where psyc ops might be a better weapon – or a gun.
An afterthought; the Lightning was reportedly very effective with guns air-to-ground in Saudi; the high wing loading provided a stable platform - smoothed the ride over hot air bumps and gusts and thus was relatively accurate – and it was 30mm.