+1 for GT grafix - used them several times for plates over the years. My requirements precisely as yours - don't want to advertise 'HALFORDS' or similar in ridiculously large/coloured font in the centre of the plate.
Although the requirement exists for a BS mark and makers details, I've had the same private plate on 3 cars now and never had issues come MOT etc. Two of the cars were registered after the date the legislation was amended - but who's to say the physical plates weren't transferred from your previous car?!
The intention of the legislation (plate makers details, requirement to show V5c etc) was to cut down on fraud from e.g. cloning, but since you can't control what is sold online from abroad it's not really worked apart from cutting down on the industrial estate cowboys - ANPR has been more effective at doing that! Basically if the plate is a 'legal' representation of a genuine reg number on a genuine car, nobody will care.
By the way, there's no set size for a number plate - just text dimensions, spacing between letters/groups/borders, so you can have a smaller plate if it has less characters in any case.
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