H21 is as always quite correct. There is a common myth concerning trade plates.
Trade plates in effect act as a portable tax disc which can be transferred between vehicles. There's a fine of £100,000 for lending your trade plates out to someone who doesn't have a just reason to use them. Only I use mine, but anyone who test drives a vehicle of mine is allowed to drive with them whilst I'm in the vehicle.
My motor trade policy is tied to my trade plates, my trade plate number has been added to the MID (Motor Insurers Database) so when my trade plates are displayed on a vehicle, ANPR or a police vehicle will firstly get no insurance up against the VRN, but insurance held when checking the trade plates.
I would simply insure the car yourself, or take out short term insurance on it. I think there's a website called day insure where you can insure a vehicle from between 1 and 28 days.