You are supposed to have the suppliers name, postcode and BS markings on there, but it is still possible to buy "show plates" without all that, or the need to produce your documents in order to get them made. Just search for "show number plates"
My mate has a garage and has a new style number plate making machine. Nowadays they are printed onto a clear transfer that in stuck onto the reflective white or yellow backing. They are not made with individual stickers applied by hand any more. The computer in the machine remembers every detail about the number plates it has made for future reference. However, it is accepted that occasionally plates are spoiled by forgetting to put the correct markings on and it is assumed that these are destroyed.