[ibrown50]

Number plates are covered by the Construction and Use Regulations 1986 (and subsequent amendments ). These stipulate the size of the numbers and letters as well as the spacing between characters and groups. The size of the plate itself is also mandated cars being larger in both respects than motorcycles. Anything manufactured 1973 or later must have reflective plates, white on the front yellow on the back. The only text ( numbers or letters) allowed on the plates is that of the vehicle registration, even dealers putting their name on the plates is technically an infringement.

Even in the correct colours the US style plates will be a pull. I’d have to check the law but I think they fall under traffic not motoring legislation so it would be a fine but no penalty points on your licence. Your best bet ( I’ve done this many times) is to get a motorcycle plate for the rear, they’re smaller so fit in the recess and although technically too small for a car legal in all other respects so you can feign ignorance if you get a pull.