I had a grey import Nissan GTR and it had the odd Japanese market numberplate slots. I had a square plate made up for the front/back and the maker went to some trouble to ask about the legality as it was slightly smaller than a standard UK plate. The character sizes were standard but they trimmed the backing plastic slightly to fit it in.
I had a few visits by her majesty's but not for bad reasons. They "offered" to make certain it was a legal import as a lot of those were pinched and exported from Japan at that time. A couple were into the Japanese car market and wanted to look as they had not seen a GTR just Evo's and Scooby's. I did ask them about the plate and they said that it was good enough.
https://www.gov.uk/displaying-number-plates/rules-number-plates
