Best be a bit careful - when my car was delivered I asked the dealer to register it on a 6-digit old reg number I've had for years, with a smaller plate due to fewer digits (perfectly legal). When I picked it up he said they'd had problems with this as the number plate mount and, I think, the area around it is very sensitive to the crash protection system. I think this launches the bonnet up the windscreen to protect pedestrians, or something.