I don't think you understand what asymmetric faction design means. As I just mentioned UKF doesn't have a reliable non-doctrinal way of dealing with flak trucks with indirect fire either. Ost also has other ways of dealing with Bofors as I mentioned besides 1. mortar barrage it with HE 2. basically get mortar HT. Just because Soviet-OKW match up is thus, doesn't mean a single Ost mortar should sh1t on the Bofors; besides OKW can counter fire with stuka and leIG fire. If there is no mortar pit, or it's not in range, then the Bofors is royally screwed and people will give up on these emplacements completely. I think you're missing the bigger picture here by focusing on a single ability which isn't super OP, and there are, as mentioned, other ways of countering Bofors.
That'd just make them into a copy of Ost and remove asymmetrical design.
stationary defensive is weak to artillery, that's how it is. If an enemy bunker down in an area, you either go around him or shell him into oblivion. Unfortunately, going around the enemy is not always an option, even in 2v2. Charging an fortified enemy defensive is suicidal, just ask any USF player in the pre-terminator days.
Even if OKW can counter advanced emplacement with LeIG and stuka, it still means the ost player is screwed without the right doctrine. The mortar ht is in a grand total of approximately two doctrines, festung support and spearhead.
The british does have a common artillery technically, the coordinated fire from the tommys and snipers. If british is going to be kept from possessing an mortar for the sake of asymmetrical balance, then at least buff the coordinated artillery. A faction with no form of artillery is going to be either a punching bag or a stonewall, and neither are that fun to play with or against. At least OKW knows to separate their emplacement and artillery.
and really, the british isn't the first faction to get emplacement in coh2. The axis had already got their flak truck and pak43 and make them sort of work.