I think that the reasoning is that the 6lber is a Pak40 with a tighter arc, which means that it should be cheaper than a Pak40. To be fair, brits are quite manpower starved, so a cheap at gun makes sense, but that is also a bad way to balance.
Part of MP starvation needs to be changed through patches.
But this one also needs proper change. It's too effective AT method at for the price that affects engagement a lot.