Clear buildings with base howitzers (Pyrotechnic tommies are GOOD), grenades, flamethrower WASP, mortar pit/bofors barrage or a light gammon bomb.
AEC isn't strictly a luchs/222 counter, and is quite capable of harassing enemy structures (OKW trucks, bunkers, caches etc) and taking on enemy vehicles. It remains useful even against medium armour due to its smoke ability and engine destroy ability and okay damage. It's even passable against infantry and is good enough to scare off squads that don't have any AT.
PIAT's require micro to use effectively (Attack ground.), but even without them you have an amazing 6lber AT gun, 30 muni mines and if a bofors can work just as well vs a 222. (Prevents him from chasing you on retreat, if not killing him outright if unexpected.)
They also have soft AT from Vickers vet 2 and Sniper.
Brits are completely fine. If you're really struggling Special Weapons Regiment has Tankhunter tommies and the godlike halftrack of doom. (Also has a crocodile if you're really, really struggling yet somehow make it to late game.)
all of the 4 other factions have a form of on map arty with no smoke, the brits really need a viable one to.
Bofors barrage has no smoke and is basically a panzerwerfer barrage with a little less damage. They also have the Sexton which is admittedly underperforming.
UKF design is perfect, only problem is units and weapons underperforming.
Base artillery needs buff, AEC needs buff, PIATs need buff, things of that nature.
These would be reasonable changes.
A conservative touch is the best way to balance this game.