Number Plate Law
Download V796 - DVLA Display of registration marks (2 page pdf file)
A 12" x 6" plate can only be used on imported vehicles which passes the DVLA criteria shown below. However, you cannot display a USA pressed plate on your vehicle, as the American font differs from the British font (New Charles Wright). In order for your vehicle to pass an MOT, the plate would need to be made in accordance with British Standards, with UK import font (64mm high - as used on motorbikes) and correct spacing etc. Below is a clipping from the DVLA website.
"All number plates must meet standard size for lettering and style (with the exception of imported cars that were not designed for UK plates and have a functional reason for not using standard size plates). Certain imported vehicles may be permitted to display number plates with smaller characters if: The vehicle does not have European Community Whole Type Approval AND the vehicle's construction/design cannot accommodate standard size number plates".
So, sadly, you cannot use USA style pressed plates on your car on UK roads. We do sell road legal number plates, but these are made of acrylic and with most registrations will have to go over two lines, you will need to visit our main site for this Craigsplates - Number Plates and Show Plates.
Notes: You can legally have your registration on one line, however,if you have a five or six character reg, which must include a number onr (half space). If you are still unsure, look at the examples below.
ABC 23 - will fit on one line on a road legal acrylic plate
ABC 123 - will fit on one line on a road legal acrylic plate
ABC 234 - will have to fit over two lines on a road legal acrylic plate
ABC 2345 - will have to fit over two lines on a road legal acrylic plate