I know there was a lot of law brought in about plates to stop the so called show plate.
i dont know the full ins and outs but i do know my plate is not legal because i have no makers mark and it has a grim reaper pic where the uk/eu logo should be. other than that the plate is right size and right font.
been stopped a few times for spot checks or speeding etc and plate has never been brought up. BUT it could be if they wanted to and were having a crack down on plates like they do from time to time.
i know mine is not legal and i take the risk knowing that i wont bitch if i ever do get caught.
I haven't seen the exact plates you refer to but the law is very simple on plates. If it is a mass produced car or bike (without complicating things) the plate has to be a standard plate conforming to bsau 145. If they confirm the plate is made to meet that standard, and shows the standard on the front of the plate you should be okay.... If not expect a £60 ticket, or worse if the bike was examined post crash (hopefully never happen but.....) be prepared for insurance company to deem it not roadworthy due to plate legality and possibly invalidate, or drop your insurance to 3rd party only......