The reg plates are 'timeless' so that if the ad is shown for a long time we, the viewers, don't all think "That car is more than six months old. Therefore I shall not buy it." Isn't it heartening to know that the advertising industry treats us all like dolts?
I recall an article in a motoring mag sometime ago regarding these. Apparently some of them are actual number plates used by request of the owner - these people write to Rover or whoever asking for their plate to be used.