Still illegal
As far as i'm aware there isn't anything in the law that states whether you can/can't have a square plate. Just that the plate has to have legal sized lettering (in the correct font), with legal spaced letters, with legal white/yellow space around it. Plenty of cars use square plates, landrovers are one example.
Same thing with metal plates, they are legal as long as they have the stamp as the law doesn't say what material the plate has to be made out of just that it has to be legal size.
Taxi cabs are another example of square plates They use them so they can mount their taxi license next to it etc.
Is that this one nipped in the bud then? Thought it was a bit strange that JDMs are allowed but UK cars cannot.