Some years ago I set up an AEC Regent 5 double decker bus for MoT.
Nine and a half litre straight six lump. Used as an advertising "tool" for the Rhondda Heritage Park, and not for carrying passengers it was class 5.
Filled the garage with smoke when I started it, but somehow it passed the emissions test too.
It was built in 1963 so classed as a "classic" vehicle and therefore tax exempt.
Provided fewer than 16 non-paying passengers were on board I was able to drive it on an ordinary driving licence.
Special build for Rhondda Transport, seven foot six wide instead of eight feet and a close ratio gearbox especially for the narrow, hilly roads in the valleys.
On a humerous note, I did once have a police escort up through the Rhondda Valley, but then the mayor, councillors and the council mace were on board.