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One thing I would argue. in tanks vs tank engagement, the 2pounder armed brit tanks were equal or superior to the equivalent early war Axis AFVs.
Where the Brit tanks fell down badly was in HE capability. This enabled many tanks to be knocked out by long range AT fire (both 50mm and 88mm) with relative ease.
Which makes me think that even if the brits had disrupted their production lines, and opted for a revised tank with saay a 6pdr gun, it would have made not a great deal of difference, except that fewer brit tanks would have been available. I have the strong suspicion that we would be lamenting the shortage of numbers in this alternative scenario, and how a few more tanks of simpler easier design might have made the difference. in other words, be careful what you wish for.
Going for a Kestrel/Peregrine tank engine would make things much better but may be too much to ask for.
I know little of Armour but see nothing mentioned of the Ram tank built in Canada using a M3 Chassis but a lower profile turret
The Ram had much merit but was not a 1940 machine. It was curiously popular 2nd hand post war though.
1940's technology turretless designs suffer from the German 'assault gun' title. They are first class mobile anti tank guns but not suitable for assaulting. Against tanks and anti tank guns they are unable meet and return fire other than from the frontal arc and, for the same reason, cannot readily support infantry with HE fire. Now 1960's S-Tank technology is another matter. Wouldn't it be good to combine that in 1940 with an almost equally out of period Mollins auto 6 pounder!
I am open minded about better cupolas on Valentines but fear raising the height too much.
I think correspondents here each have a different philosophy on the possibilities and swapping technical numbers will not change these. Mine is simply to stick to something we know will work and to approach it from an industrial point of view(ie make lots of something just adequate).
HE weaknesses in my Valentine were the reason for my 6 pounder gun (or Tony Williams'). Certainly a 3 man turret is far better than a 2 but 2 did work and you can get 3 into a Valentine turret ring (see Crusader, AEC and Matilda II 6 pounder turrets). As for speed, I am not sure this is as important as others do, but ungoverned Valentines could (and did) run at a reliable road speed of about 30 kmh which is much the same as early PzIII or IV.
The opposite approach is to seek the best technical device which became the German one and left the PzIV still making up much of the numbers in 1945. The best drove out the good. The consensus is that the Germans would have been better off if they had either stuck with purely PzIV or moved on entirely to PzV. Like the British they played around with too many alternatives. The Russian's and American's T34 and Shermans could be criticised but they went for huge amounts of them as a standard item. For the British and Commonwealth forces in 1940 what I believe was needed was quantity production of a single tank type that works. For my money the Valentine answers this, even if it could be better in various ways.
Incidentally, the Kestrel was out of production long before 1940 and Miles had to search around for as many as possible for Master trainers until they had to move on to Mercuries. The Peregrine was a dead duck in 1939, only being made to use up Whirlwind production. The Rover Meteor took until 1943 to get going and 1944 in quantity (even then having to re use old Merlins to keep production going.)
The AEC was a sound engine but low power (130-160 bhp). The GMC replacement went up to 210 bhp. The situation was that UK did not make large enough lorries to have a bigger simple 6 cylinder engine but USA and Canada did so that is what went into Canadian then UK Valentine production. Australia and India(?) also had such engines and there is no reason why they could not also have made 200+bhp Valentines. India certainly had the railway industrial capacity to make Valentines.
As a last piece, I had cause to speak with someone whose father was a Russian tank commander 1943-45 in Valentines and Shermans. He allegedly much preferred the Valentine because you could hide it. He compared it to infantry and cavalry. If an infantry soldier is shot at he can lie down. The cavalryman is stuck 1 metre up in the air.
For those that differ from my take on the original question; I readily acknowledge that you can postulate a better tank than a Valentine, but do you know if you can make it in quantity and in time using the actually available industry?