I've got an 8" x 6" plate on mine, which is smaller, but not taking the p155... Come MOT time, I will have to get a 9" x 7" plate again... The lovely people at DVLA have standardised
everything, so anything with isn't 'Standard' is illegal
Keep your old plate & get y'self a smaller one... You'll just have to swap it over when the time comes, then swap it back
Beware, Ol' Bill can 'have' you for a smaller plate... So be careful