Plates are a weird one, some people get chopsy when stopped by the old bill, who will then look for a reason to give the person a bit of paperwork. The easiest solution is a ticket; this has a financial penalty but the matter ends there. The police can if pushed seize the plates. The next is the Vehicle Defect Rectification Scheme. This means a telling off and a notice to rectify as soon as practicable AND getting a form stamped at an MOT station to say it has been done, then producing at a cop shop. The ball ache with this, is that if a copper stops you the next day with the plates still on, they will either give you a ticket or report you for summons. Legislation actually gives font types and sizes. Any deviation from this is an offence.