The 3" and 4" mortars were purely using the UC as transportation, however the 2" version is as I suspected something that can be set up and fired from within the UC. I wasn't 100% sure of this earlier today, but now I'm reasonably confident:
Interestingly the 2" mortar was not only fired from the UC, there was a permanent mount in some UCs for it. So it was truly a Self Propelled Mortar, unlike the 3" and larger versions. Apparently the Australian 3" carrier was a specially modified version that could fire while mounted or dismounted, however that's not in British use.
My earlier concern it was unhistorical has been wiped. Hmmm, maybe this could be an interesting change while making Been Carriers relevant in the meta as well.
Certainly better than the mortar pit that we have now, give it normal mortar accuracy but with a mortar fire/move toggle (can do without, just a balance suggestion on the safe side) and this would be great, probably would cost 80 muni.