I have an import size plate on the back of my car.
From what I understand the size of the plate is irrelevant as long as the letters and numbers are the correct size and font and spaced correctly, and I believe mine is the correct font, but has slightly too small letters to be fully legal.
However, because I use my car to deliver pizzas in and work just up the road from a police station, I get pulled over all the time, normally atleast once a week, usually to check its insured properly for business use etc. Not once have the police mentioned the plate, and I have been pulled a couple of times by traffic cops which are the type i'd expect to mention the plate.
It's also passed two MOTs with it on, so i'm guessing unless you were unlucky enough to find a cop that really knew every detail in the law, you'd be fine with it on