Under the Road Vehicles (Display of Registration Marks) Regulations 2001, if the police spot anything on your number plate which reduces the legibility of the plate, they can impose a fixed penalty fine or prosecute you. (The maximum fine a court can impose is �1000).
So you could be driving at a legal speed and get stopped for something as simple as having a tail light out. When PC Plod walks around your car, looking for anything which he can nick you for, he spots the cling film. He'd almost certainly impose a fixed penalty for the number plate offence and he'd probably be far less likely to let you off the tail light offence.
Cling film doesn't hide your number plate from safety cameras but it does increase the chances of you picking up other fines.
Chris