https://web.archive.org/web/20100414023110/http://www.royal.gov.uk/TheRoyalHousehold/Transport/Cars.aspx
>The Bentleys and Rolls-Royces uniquely do not have registration number plates, since they are State vehicles
Diplomatic cars have special plates that differ from the usual format, but it appears that State vehicles don't require them. As you say with reflection glare/distance/low res it can be hard to tell, but it does look like this silver estate (Audi??) doesn't have rear plates.