2.5 inch is also the font size for car imports, which mine is. And as I say a standard UK plate size does not fit, therefore a smaller plate is needed, therefore a smaller font to permit the required border around the text.
Wow useful info. Jon
I've found this which explains quite a bit:
https://www.craigsplates.com/import-number-plates
So if for example you buy a grey import 350Z with a rectangular rear plate this is for you.
All 240/260 rear ends are made for foreign countries tbh and all imported from Japan.