if you read the rules theres nothing that says it cant be behind glass? just needs to be legible and correct size etc.
but then again if your coppers a cunt im sure he can delve deep and find a reason to do something bad.

but generaly worst come to worse its a £30 quid fine, no points or anything. ive been pulled loads of times for my german plates on the beemer and the small plate on the fez but they just tell me off, i asked once and he said you dont get points just £30 quid fine, and he said theres no limit on how many times either, cheers coppa