Units are balanced only within the context of their own faction. The unit cost of the Jackson doesn't matter for the unit cost of the Firefly.
Again, I do see your point. Still you didn't answer my point. You said yourself Firefly was lowered in population because of the high population costs that UKF has in general. It was balanced within the faction. That is true. But in addition they are way more manpower starved than USF. So 440 MP for Firefly is a lot more than 400 MP for Jackson. In fact it feels more than a 40 MP difference for a tank that isn't anyhow stronger. So shouldn't that be balanced within the faction too?
On top of that, the Jackson artificially overperforms a bit on purpose because it's the only reliable/heavy AT that USF has in the late game. UKF has the 17-pounder, Churchills for meat shields, Comet, etc. to help hold back Axis' heavier armor.
About the Jackson: True, USF just should have a nondoctrinal alternative, that would solve many problems with USF tank roster and the status of M36 as the "Holy cow".
But about UKF I wouldn't agree.
Churchill: It is just a meatshield that is fine versus infantry but deals damage to other late game tanks too slowly. Meatshields are not that valuable in a game where you can choose your targets freely. Having one means you have one Firefly less, in tank combat like Panther vs Firefly it can pretty much be ignored until Panther gets real low on health.
17-pdr: Only a problem in low skill games. Limited placement, big hitbox, easily counterable. Often enough just a waste of ressources.
Comet: Better suited for tank combat than Churchill for sure. Strong versus infantry, pays a price for that when fighting the heavy tanks and Panthers. More like a fast allrounder with decent hp/armor.
So, Firefly is the only reliable/heavy AT for UKF in the late game too when it comes down to dps/penetration. The other tanks surely can assist, but if you want to fight heavies reliable, you better go for double Firefly as you would do in a similar way by playing USF with double Jackson.