I cannot guarantee that this method still works but it worked for my father back in 83 bringing a bike from Cyprus back to UK.
He popped into the the Movements staff at Akrotiri to enquire about how to do it.
When he came out, the bike and two crates of beer in the boot of his car which he had "forgotten" to lock had been "pinched".
Rather than getting the RMPs involved, a little time was allowed for it to be sorted "internally".
A few weeks later, a phone call was recieved informing him that the motorbike had been "found" at Brize Norton and could he please come and collect it.