It is illegal as steve said, depending if its a private plate or not, if ur pulled the police can fine you for it and take the plates off you. If its a private plate the police can request the DVLA take the registration back off you (you usually get 2 warnings first tho). Chances are you will get pulled over by the police, just depends what the copper is like at the time to how bad ull get done. If your going to put a Black reg plate on i would recommend you carry a legal set of plates in your car aswell, so u have them to put on ur car if u do get pulled.
I personally have a private reg, i have 1 more warning before my plates taken off me from the DVLA, i dont fancy paying the 4 figure price for it again tho
Im guessing uve seen pictures of charlies cooper:
The plate does look great.