Of course they are illegal.

According to the law, number plates should be made by an authorised establishment, with the correct lettering/spacing etc and with the name/postcode of the company that made them at the bottom. If the Fed's stop you and the plates are not in spec and they have the makers details on them, they are in the shit. If they don't have the makers details on, you'll get a VRN I'd imagine.

Of course, people do ride around with illegal plates everywhere, so try your luck?