No they are not traffic cops, but all the traffic cops have got to go on is their car based computer system and it will tell them that, in my case my number should be on a Silver A3 Sportback which it is. If the police have any registration number queries it is the DVLA who have the final word. The Police ANPR system is not updated that quickly. I suppose they could ask to see my V5 and check that the VIN number and registration number match, but I don't carry my V5 with me, so no doubt I would have to present it at a police station within 14 days, by which time everything would match anyway and providing I'm not breaking the law in any other way I doubt if they would even bother. The other way of dealing with the change over of the number is to let the garage register then new car with a new number and then apply to have it changed to my personal plate once I have the new car. I have done this once but it seems to take a lot longer and you are driving around for a while with your new car but without your personal plate. I prefer to do it my way and save the cost of a set of plates and the admin fee that the insurance company usually charge to change the registration number on a policy and then change it again few days later with the details of the the new Silver A3 Sportback and have my personal number on the car when I collect it. I don't apply for the Retention Certificate until I know the new car is at the dealers. It only takes a few days and as soon as it arrives I take to them so that they can register the car with that number, which as they now do it online only takes less than a day. By this time the new car has already had it's PDI and all it needs is it's number so we agree when I will collect it and they arrange for the Audi complimentary 7 day insurance and I tell my own insurance of the change of car within those 7 days. All very simple and straight forward.