Many Police cars are fitted with ANPR (Automatic Number Plate Recognition) and can instantly tell if a vehicle is taxed, MOT'd and insured. Quite a good system and it works automatically. However it can take a few days for a new car to be on the system for things like road tax and insurance...
As long as the numberplate is 'readable' by the ANPR technology, I doubt you'd get a stop for a metal plate.
