Until I take my car to the dealers it still has a valid number and insurance. My current insurance company is not covering the new car as that is covered by Audi's complementary 7 days insurance. When the DVLA send you the new number for the existing car it says you should change the plates "as soon as possible". No actual time limit is given and my dealer has always said they would not consider it a problem as both cars will never be on the road at the same time. If they need to take the new car out as part of the PDI they would always do so on trade plates. The dealer does not register the new car with my private plate until the morning I am collecting the new car and at that point I am driving my car to get a new set of plates. Then my private plates are taken off and if they need to take in out at all they again do so on trade plates. The strange thing is seeing two cars at the dealers both with the same number and I often take a photo of the two cars together.