British plates have never been made and issued officially as in the U.S. The appropriate authority assigns the number (local councils up until the 1970s, the DVLC/DVLA since then) then it's up to you to get the plates made up and fitted, which is why car accessory shops (including Halfords
) make them. Dealers normally provide the plates for brand new cars before sale.
The fact that the plates are merely for displaying the assigned registration number and don't perform
quite the same function as American plates means that they are traditionally referred to as simply "number plates" rather than "license plates."
There are rules about the size, spacing, and type of characters and the colors required (which vary depending upon the age of the vehicle), but until very recently there were no restrictions whatsoever about making them. You could just walk into anywhere which made plates and ask for any number you liked. That changed a couple of years ago, and shops are now required by law to see proof of your identity and proof that you have some entitlement to the number in question. The same applies to mail-order suppliers.
This applies only in England & Wales, however. Scotland is still completely unrestricted in this respect, which rather makes a mockery of the whole "defeating counterfeit" idea behind the change of law when you can still get exactly the same plates sent by mail-order from Scotland with no questions asked!
By the way, on imported vehicles you are allowed a smaller-than-usual plate where one of the standard British square or long plates won't fit (as will be the case if you have a recess designed for a regular 12x6" U.S. plate).