You could always get round it being "illegal" if you really believe it might be-fit the standard plate after spraying the aperture yellow :-)
I think the post is referring to Nich's plate, which has an MG logo where you might have GB/Euro badge. A standard large rear plate is not illegal, it was designed to have one.
What area of the country are you in?
SJR
Super Jumbo Racing 05 260 Saloon 87 5.0 Mustang G.T.