I'm just going to cut my existing plate down to fit in the new JDM rear bumper.
I can do this because it's only a 5 letter plate which is to the correct size letters spaces etc. It doesn't say anything on the Gov site about plate size just letters, font, style and spaces.
If the plate itself had to be a specific size I.e. 20.5 x 4.5 then any car where the number plate was irregularly shaped or larger like the Rover 75 and some jags would also be illegal?