There is only one problem, and it's called bofors. Mortar emplacements are ok , like at emplacement , german factions have one too. But bofors hold an area by infantry, light veichles and suppress infantry with indirect fire. Brits is a good faction anyway without bofors, it's only an easy way to def a sector and build the simcity. Brits need to def mortar like all other factions, mg, infantry and fhq. It's enough. Maybe you could give them a classic mini flak position.
While mortar pit is an unique unit that is hard to balance because there is no similar unit on axis side, Bofors is a pretty straight mirror of Schwerer HQ. Both units lock down an area for infantry and light vehicles completely, in case of Schwerer some medium USF vehicles are affected too. So while having different stats and abilities (and you could discuss a lot about that) they fulfill the same role of area denial while both are not that easy to kill when supported correctly. Both are repeatetly used to lockdown important VPs or fuel points. Do be fair: If you want to address one you have to address both.
You can get your Bofors up earlier but this comes at a big disadvantage: While Schwerer HQ is your tech requirement for medium and heavy tanks, the bofors delays your tech to tanks. Use that to your advantage. Brits really need their tanks to fight enemy armor, inspite of having the best AT-Gun around. Their problem is that their infantry can't rely on a decent handheld AT-weapon like Shrek (Piat, cough...) or any kind of vehicle snare to slow axis armor down in front off 6pdr. You really can put pressure on a brit with Bofors by going for a fast tank tech.
So I still think, when something has to be changed, it is the mortar pit.