Strip the canvas off the Centurion mantlet and it looks similar to the Comet. Take the Comet and put a Cromwell turret on (in your mind only) and you can now appreciate that the Comet was the Cromwell if the turret and gun designers had actually talked to each other. The Centurion was the Comet freed from using Cromwell jigs (hence the sloping front) and general agreement that Horstman bogies were as good as Cromwell suspension but infinitely easier to maintain and replace.
For a mid war British tank one needs to define your 'mid war'. Winter 1942/3 fits the bill to my satisfaction. So you only have Valentine, Churchill and Crusader to choose from. The Meteor was first tried in a Crusader in 1941 and the Crusader replacement was to be a modified Crusader hull with a Meteor engine and a 75mm HV gun. As it happens they were still manufacturing Crusaders into 1945 as 17 pounder gun towers. Even the Crusader turret could take the 75mm ROF gun. The Churchill was beginning to move out of it's unreliability stage and could also go on to mount the 75mm ROF (if you can put in a 6 pounder you can re-barrel to the 75mm). The Valentine was able to finally mount the 75mm ROF but only in a 2 man turret but fought it's way to Berlin.
If I had to choose from what Britain produced in late 1942 for delivery in early 1943 I would have to choose the Valentine. More reliable than a Churchill (then) and a Crusader. Slow yes but it can mount the same gun as either (albeit in a 2 man turret). Cheap and easy to make. Now if I had to make the same choice but to use them to the end of the war then the Churchill's armour would tempt me.
One might take a lateral approach and note that the limit on British armoured unit numbers was that of crewing and supplying them. By 1944 Britain had access to more gun tanks than it could field. One could postulate that investment in more transport for the logistics train, more transporters and increasing armoured crew training might give better returns. Of course even better if they had better tanks. Equally one might opine that better tanks meant that you did not need so many so you can make do with a poorer logistics train. If I were a tankie I would opt for the comforting armour and escape hatch of the Churchill.
While I would agree that the Churchill is probably best around 43, I can't believe that there's been so little mention of the Matilda. It was slow and not heavily armed, but it could take a pounding and was in service until the end of the war. Notionally the Valentine was built as a replacement, but it was about as slow and had less armor, so it's main benefit was it was cheaper per copy.