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.
The problem is: there is not a disadvantage. Bofors cost only 30 fuel. You can spam 3 bofors without delay so much your tanks, need only one or two at guns near your simcity and tanks are stopped. Now there is a counter battery for indirect fire. So counter sim city is too long, and we must consider that probably while we are trying to take down bofors (covered by mortars) , fuel or vp are holded by them. So not a big problem for brits late game. Okw flak cost 120,a lot risky build that on aggressive position (the only problem is that everyone that lose tier 3 can go for kt anyway) and comes later. And it's only one. It seems not a big problem. Bofors spam is cheaper. 3 bofors 90 fuel + bofors fuel tech? Crazy. You lose one bofors? Np , build another one. There is no punishing for bofors spammers.