My old tester always said to fit a normal plate for the MOT, after all you never know who is sat outside the workshop. Never had a problem with doing that for him as he was a sensible tester. Luckily my current plate is 100% legal despite being smaller due to the reduced number of characters.
I've just bought a bike today with .. HRC , never had a private plate before. Only thing is it's a Repsol and not in HRC colours.
K777 CJS is my bfs plate, but is on my daily car, S777 JCO is my plate, and is on my astra turbo (broken) so the K777 JCO fits in nicely