My corsa came with illegal plates on, wrong font, wrong spacing, and one of the previous owners names written below the reg. I argued with the woman who sold it to me - it added to the points I pulled the car up on to knock the price down, knowing that £30 would be the cost to replace.
Then, I went on eBay and ordered a set for a tenner. Thing is, if your car is ancient (like my Corsa) then how does anyone know when they were replaced? As long as there's a BS (british standard, not bull...t)number (forget which one) then I don't think there's any bother.
It's a pet hate of mine. I don't understand why the ANPR cameras aren't dishing out hundreds of fines a day. It's not difficult - right font, right spacing. It's patently obvious when it's wrong. It's the law. 99% of people manage, yet I could be well into double figures of 'offenders' every day if I bothered to count them.