no one has made a 1/56th challenger A30
There is a JTFM one in the pipe, but I wouldn't like to try & put a date on its release (here's a pic of the tracks & running gear attached to his Cromwell).
A Black Prince would be very cool though...
If we're indulging in a little whatifery, how about the Charioteer (I know it didn't appear til 1950, but exigencies of war & all that..) It's a slight puzzle that they could fit a new turret mounting a 20pdr/84mm onto a pretty much standard Cromwell hull (the turret ring was widened slightly) but not have figured out how to do the same with the 17pdr a few years earlier (the 20pdr is a lot bigger in all dimensions & although the turret was a bit of a bodge - tall & fairly lightly armoured - it's not too different in concept to that of the Challenger). A charioteer would also be a far easier kitbash & scratchbuild, as you'd only have to come up with the turret/gun - a fairly simple shape.