Chatting to the Oshkosh crew, I learnt that they are based at Bulford. One of them said they are part of an "agency", and get the call to transport tanks from time to time. Presumably the civvy registration would show that they are civvy owned vehicles. The Challenger, not the proper recruiting team one, was "signed out" from the stores!!
PS if they were Cornish ones they would have WK or WL registrations!