. .I have lived in Hawaii, Rode Island, Oregon, and California, and have had vehicles in each place.
Each time I needed to get plates I was issued a plate from the government that had standardised hole spacing, fonts, colours, size, and if 2 were required (some states are one plate and some are 2 plate) then I got the correct number too. These are made from an alloy, covered in high quality retro-reflective material, and last ages. Should I want to register a private plate then all I had to do was go to the DMV and ask if it was available and pay a small fee.
In Hawaii I had ONWARD on my motorcycle, cost me $25 extra per year. On my Jeep I had mounted my plate to the front bumper and had bent it around the top so that it was not damaged when off roading.
Perfectly legal. I will never understand why the English government seems to delight in making EVERYTHING harder on its citizens.
Mike