The issue is the UK really needed, either a clean sheet design that could easily accommodate the 17pdr, or an intermediate tank gun that could replace the 75mm while providing superior anti-armour performance to the 6pdr. It all seems to come back to some incorrect assumptions made during development in regards to the size and platform impact of the 17pdr, that had it been realised would have resulted in design changes to address the problem.
Enter the Centurion. A clean sheet design and the first complete one undertaken by the Ministry of Supply's Department of Tank Design, rather than the individual manufacturers (the Department had up till that point acted as a contracting department, requesting the individual manufacturers to design the vehicles themselves). The Department was much closer and more clearly understood the requirements of the end users and so the Centurion incorporated the lessons learned from the previous five years of warfare. It was intended to fulfil both cruiser and infantry tank requirements. In the end, the infantry tank version wasn't built, beyond a few prototypes (it incorporated heavier armour and wider tracks IIRC).
You should also remember the 17 Pdr was designed first and foremost as an anti-tank gun, not a tank gun. Only after it's development was the idea of putting it into a tank first mooted. The British suddenly found they didn't have a turret big enough to accept the weapon and so it became a seemingly endless chase to either adapt or built a new vehicle. Vickers tried to fix that by developing their high velocity 75mm gun, which eventually morphed into a 76.2mm weapon firing the same ammunition as the 17 Pdr with a shorter, fatter case and was finally designated the 77mm (despite actually remaining 76.2mm). While it's velocity wasn't quite as good as the 17 Pdr. it was close enough for most jobs.
An argument could be made that the Centurion was the first successful design to be able to mount the 17 Pdr properly. In that case, the decision made very early on was to give it as large a turret ring as could be managed, which meant it was able to survive the succession of up gunning, from 17 to 20 Pdr and finally 105mm. The Firefly was only ever an extemporised weapon, forced on the RAC by circumstance (and the happy coincidence that the RAAC had done the experimental work with it's twin 25 Pdrs mounted in a Sentinel in Australia which proved it was possible). As it was, it was only accomplished by turning the gun on it's side. The Black Prince, was the last gasp of the "old school" and they failed as they usually did by trying to do more with less and failing to produce a balanced design as far as it's engine power went.
One thing I've never quite understood is why the Allies never followed the German route in seeking a higher velocity tank gun - lengthening the barrel and increasing the chamber size of the standard 75mm tank gun. It would have made a better tank killing weapon without necessarily needing a new mounting. As it was, they stuck with a gun which was adequate in 1942 until the end of the war, when it was decidely passe. While the primarily role of the tank in US doctrine might have been infantry support, a higher velocity 75mm would have taken little from that and ensured the tanks had a better weapon with which to defend themselves against other tanks.