A 7x5 plate will cost you £30 when stopped by the law.
A second stop with the same plate on, and they can confiscate your plate, or take you to court, because you've had time to rectify it.
HTH.
EDIT: Also, officers in a car are not the one's who decide if a persuit is a go, regardless of who they are persuing. That's the job of someone in the control room. A car can happily persue a motorcycle, however, the second that a biker removes their helmet, general concencus is to terminate the persuit on the grounds of safety.