From run ins with the police while driving around with mates who drive imports, the rules for them seem to be fit whatever you can in the factory location that looks sensible. Obviously if someone tries to convince the police their import Civic Type R ran 300x45 plates factory they're gonna get hassle but something reasonable ish generally tends to fly. No idea on the full legal standpoint.
As far as the Clio slim plate, I've been pulled various times, and not one officer has mentioned it. That's even when they're staring it in the face to run a vehicle reg check! The only time I've ever been questioned about it is when I went to help a mate who had an accident. I parked up at the side of the road (not involved in the accident, I arrived a good twenty minutes later after a phone call) and the traffic police (the angry ones in X5's) pointed at the Clio, asked who's it was, and strongly recommended I fit a proper full size number plate.
Being honest my front plate is on Velcro, with a full size one also on Velcro hidden away in the car somewhere. If any officer did want to issue a fine I'd certainly offer up the instant fix in the hope of getting let off...
