yep sure is illegal, couple of years back i had my plate made custom in a very thin perspex, got pulled over for defects and the cop didnt notice that the plates werent standard rta issue till he asked me why was there a corner missing off my numberplate, i told him i scraped it somewhere as it was low to the ground, so he kneeled down to have a look as he grabbed it he snapped it staight in half, i put on my shocked face :nowigetit and told him i bought the car like that, still got a fine and defect. no more cheap perspex, gotta go custom 10mm next time.