You're interested in regulation 14a on page 2. That shows you the required dimensions for the lettering, borders, stroke and all that good stuff. Technically, it's a black and white regulation and it's generally written for cars like ours where the rear plate recess would prevent a normal size plate being displayed.
It becomes grey in its interpretation with our front plates which could house a standard UK plate. If you fit the front with the square "import" shape plate, I doubt you have any problems, but use the same size lettering and grouping them on 1 line means you can fit the whole registration on a 3/4 size plate. That's what I have done (when I actually have a plate fitted!)
A member of a UK Supra club won a case in court arguing that using a square import plate or a UK size plate, restricted the airflow into his radiator and allowing him to use the same size plate as me.