Being able to mount a crew to fire a mortar and firing within the truck are two completely different things, I simply don't think the UC was large enough to support a mortar crew firing from inside, which was what I meant.
Something that could be done - not sure about its gameplay impact - have a mortar that can be unpacked from a UC to have 1/3-1/2 of the hp of the current mortar pit but take 5-10 seconds to unpack/pack. That would be certainly more interesting and less gamebreaking than the current mortar pit.
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.