The idea that steel is not retro-reflecting so cannot be used for number plates is an internet myth as there are no number plates made from retro-reflecting material - ordinary ones are non-reflecting plastic (acrylic, I'm guessing) which has a retro-reflective coating (the white or yellow layer) on the back. Sticking a retro-reflective coating on the front, as most Yurpeen pressed plates have, has the same effect.
But to be legal in Britain, number plates must have the makers name and postcode in little writing at the bottom which allows the maker to be traced. No little writing (or 'for show use only') allows the Old Bill to do you if they have you stopped for another reason - they aren't likely to see even if tailgating.