I have seen and heard of pc plod stopping cars with metal number plates. They then had to have them made in perspex. I don't see anything wrong with metal number plates, but the traffic plod do.
The other thing I have seen traffic plod giving folk a tug for is the use of football club crests at the end of a number plate. Apparently the only things legal on a number plate are the registration mark, obviously, the BS___ code of the plate supplier, supplying dealers name on the bottom of the plate underneath the registration mark. The only flags allowed on cars registered on the UK mainland are the European blue flag with the stars in a circling the letters GB, or St Andrew's the Saltire, St George's cross and the Welsh flag. I would like the Yorkshire flag, but have been told I am liable to getting a tug from pc plod or traffic plod.
Saw one guy in a VW with metal number plates getting grief as he had 'D' not 'GB' on the plates. I can see why, as the car was GB registered, not German registered.
Over time I have seen several vehicles with wonky number plates. I put it down to the use of adhesive pads holding them in place, rather than self tapping screws or plastic bolts.
Sorry to have gone on a bit!