Centaur
FF
QF 17p
Heavy engineer
Churchill
UKF M3
Some remarks:
QF 17pd
Later: yes
Less range: Definitely not. It should get slighlty more range than elephant/Jagdtiger to not get picked up by these. With 10 range less this would be even more hilarious.
FF
From all allied TDs it is the one I wouldn't discuss to change anything more. It has its downsides with very low RoF, low speed/acc and very unreliable rockets. Some more turret speed is a good change. Its not too good, really doesn't need a nerf.
Centaur
No, I would see the Ostwind on top. Reason: survivability. You are overestimating the changes. While Ostwind can escape some of its allied tank threads Centaur still gets caught up by units like PZIV, Panther or StuG to get that one or two extra shots off and destroy it. Ostwind can escape more easily. In addition Panzershrek pens Centaur with his far superior armor 100% frontally at long range, that is not the case with low armor Ostwind and all allied infantry AT weaponary. And while Centaur has no access to smoke Ostwind really gets a panic button with doctrinal Panzertactican. All told Centaur has a harder live. It is still easier to vet up an Ostwind.
Churchill
Just want to point out that it says nothing that it is called a "triple buff", because two of them are very small (RoF and population). A low angle rocket attack for Panzerwerfer is only a single buff for example but a really big one. Just for comparison that "triple" is worth nothing here.
I would reduce its price and population even further, nerf damage and penetration and give it slightly better AOE and strong MG damage. So it would be much more of an infantry killer with consistent damage output that really struggles to keep up in a fight with an Ostheer PZIV. The infantry supressing Panzer Shock ability in CoH1 was a funny one, would have loved to see it here too.
Thick smoke is far better, to assault MG supported positions. I wouldn't change it.
UKF M3
The commander is pretty dead if you ask me since the Rifle boys are not allowed to be upgraded anymore with heal/recon and Sappers got an AT grenade. For a mobile halftrack you are better off with Land Lease commander and its M5, because he has better abilities in overall.
IS and Sappers/Heavy sappers
The basic problem of UKF is that is basically only has one combat infantry unit. Even USF with Riflemen centrism has more, because Rear Echelons have the very useful smoke nade, a suppression ability and are a nice cheap AT unit, when equipped with two bazookas. In addition Rear Echelons are not very busy with engineer jobs, because of the absence of real mines and the existence of USF vehicle crews for repairing. Sappers on the other side are very busy with laying mines, building emplacements and repairing vehicles/emplacements. In their sparetime they are a cc unit theoretically, but they can't compete with other real cc units, so screw them there. Heavy Sappers with Vickers and two Brens are not worth it either, because it costs way more ammunition than getting IS with two Brens which inflict similar damage because of better long range stats and additional Enfields instead of Stens.
Last but not least you get Sappers when you already have 3-4 partly vetted IS squads, so there is no reason to build multiple Sapper squads.
So IS is forced into beeing the jack of all trades even more than Riflemen. The only way to change this is either drastically change the combat role and timing of normal sappers or add a second nondoctrinal Infantry unit. Then you can take away some of IS abilities and allocate them.