Someone had better tell The Queen that her various V8 Rovers and her Landy (amongst others) belong to The State.
Similarly Prince Charles' various Aston Martins (DB6 Volante, V8 Vantage Volante etc., etc.) and numerous other makes and models (Jaguar, Range Rover, Bentley etc., etc.) in his private collection should be returned to The State forthwith.
The Royals may be afforded the use of bomb-proof Bentleys and the like for state purposes, however they certainly possess their own private vehicles (with their accompanying registration plates).
BTW The Queen has the plate 'A7' on a 1900 Daimler, although it is unlikely that you will see her driving it on the public highway.