I actually think the way to go with the Cromwell is improving its AT: give it timed +penetration rounds or something.
If you improve its AI, it competes with the Centaur and the Churchill. What UKF really needs is a competent AT tank sitting between the AEC and the Firely.
Maybe move some of its speed buffs down vet, so it can disengage really easily?
that is much as a argument as for the p4 (ostwind and stug)
it already has the best AT of all medium tanks (50% reload at vet 3, best pen profile with high pen at all ranges, commander give extra accuracy and sight)
i would prefer a little buff to the mg too, to kill stranglers when u hit them with the 75mm