I'm sure you will get a pile of replies, with dates, but as I understand it, right at the start of the war, British vehicles had what was effectively an ordinary civilian registration, and some had army registrations as well. Shortly after that they dropped the civilian reg and just used the army one, but note that the 2/2/2 system is MUCH later, although some vehicles that stayed in service, like DUKWs, had a USA number first, then a wartime British army registration, then got renumbered in the 2/2/2 system when that was introduced.