Thing is Bofors is usually going to get taken out by indirect fire or by tanks. 2X LeIGs usually hardcounter it well (assuming no standfast which is more cancerous than advanced emplacement upgrade). As Ostheer, currently you'll need to split up mortar teams to avoid driven off completely by the suppressive fire, but if it's removed, then there is absolutely no counterplay by the UKF. Perhaps we should also consider the relatively cheap cost of the Bofors.
you don't see the problem in forcing the ostheer to get two mortars? Artillery are suppose to be a hard counter to stationary defensive. bofor shouldn't be able to fight back. Both the flak truck and mg bunker completely lack the ability to fight back against artillery.
British lack mobile artillery, yes, that's why they need an actual mortar instead of mortar emplacement.
3 isgs don't defend themselves against infantry and light vehicles though. Otherwise good math
three isgs can actually defense themselves against infantry pretty well. the long range of the ISG's auto fire is pretty potent.
even without the suppression ability, isg + flak is a pretty formidable defense. The combo would be a lot weaker if isg's auto fire range get cut down to 80 meters, but that would probably require a cost decrease as well.