the 6 Pounder to QF75mm is only a new barrel so by 1940 the gun going into new British tanks if the ROF QF 75mm gun with good HE and capable of killing any tank on the planet. All it needs is a reliable hull to carry it around.
This. Realistically, any of the guns mentioned upthread would do the job in 1939-43. Whether we're talking about the 6-pdr with HE, the 13-pdr, the 75mm or some version of 3", it's going to have a useful HE round and enough AP punch to handle mid-model PzIII/IV, never mind Pz38t, PzII and whatever trashcans the Italians put out. The issue is getting it on a viable tank and getting the tank to the front in numbers.
As for the Valentine, if you offered it to me for the British in 1940 i'm biting your hand off. if you offered it to me in 1940 with the 3 man 2 pounder turret I bite your hand, arm and shoulder off. The problem is it really does have limited room for growth and is too slow. By the time you are into 42 it is pretty much out of contention and past that not worth it apart form as a hull for things like the Archer. The thing is though make it a bit bigger so it can take a 3 man 6 pounder turret and make it 5mph faster, 10 if you can manage it and it is perfect until late 42 and still really good until early 44.
Also this. The idea of building a standard chassis that will remain viable throughout the war and can be used as a basis of a family of TDs, SPGs, APCs and whatever - a British equivalent of the PzIII or PzIV - is a very good one. OTL the British wasted far too much time and effort on a huge multiplicity of designs, mostly built in small numbers and then discarded. The trouble with the OTL Valentine is that it is
juuust too small to be that chassis. It was solid enough when it came out in 1941, but it was always slow, the armour could not be upgraded, it took a shoehorn to get the big gun into the turret and the ergonomics were correspondingly poor. There's a reason why the British retired the 6-pdr Valentines quickly and the 75mm version never reached the front at all. Plus the story of the Valentine derivatives isn't great - the Bishop was a bundle of design flaws that was quickly discarded once the M7 became available and the Archer was only suitable for the defensive battles the British were no longer fighting.
What the British need to do is get a chassis in the 20 ton range (with the potential to go up to 25t later), find an engine that will move it at 20-25mph, pick a suspension (doesn't matter which, so long as they test it enough to get the bugs out of it), pick a gun (again, doesn't matter which so long as it fits comfortably into a turret on top of the aforementioned chassis) and get the whole thing into volume production by 1939, so it is coming off the line in numbers in 1940.
What they don't need to do is get into a massive turf war of my engine vs your engine, your suspension vs his suspension, my gun vs all your guns, everyone's theory of tank operations vs everyone else's theory of tank operations and hit 1938-9 with a bunch of dodgy prototypes, some of which are then ordered into panic production once they realise the war's starting and no-one's brought the tanks.