(long time ago) i used to do install work for a bus that had been converted into a travelling art gallery, I converted one for similar purposes too.
The gallery one was operated by a council and could be driven by anyone with a standard drivers license and a certain amount of chutpah (this was pre 1997 mind – only people with older licenses could do it now), in much the same way as a 7.5t truck can be/ could be. I don’t know whether there needed to be an ‘O’ License held by someone, I wouldn’t be surprised if there wasn’t though.
I only had to move the ones I worked on around my yard, rather than on the open road – its interesting, pre-selectors and nonsense like that. Its like a ghost is driving the bus you you’re trying to stop it.
Depending on how its reclassified the size / weight would be irrelevant to HGV or Operator License rules – if its not being used to carry goods – some SUVs are heavier than laden commercial vehicles, and so called ‘Dual Purpose’ vehicles (Navara type 4×4 pickups) can dodge the train-weight rules when towing and have an all up weight that would require an ‘O’ license with any other kind of vehicle
The buses I worked on had all the seats out, and in some cases had the top deck floor removed to make a larger exhibition volume. That means they had probably been re-classified as another kind of vehicle.