They usually do both.....I know Craigs do legal and show.
If you want legal, you can't put anything on them, they have to be specific sizes, text font/size and colours.
You will also have to show/send in your driving licence.
You can buy personalized registrations from the Ministry of Transport/DVLA or other companies out there, but they're not cheap, otherwise you get what you're given from the DVLA.
As your car is pre-'73 you may have black and silver or black and white plates.
As your car is US you may have the smaller size plates with smaller (motorcycle) size text to fit the smaller plate areas.
Read all the details on the site, then you'll know what to ask for.