I used to work in Halfords, many moons ago (well about 13yrs) when i was a young and spotty, and i can confirm that they only produce leagle number plates in store – if you require a pair they will/should take some details from you (name, car type etc).
The number plates are full BS compliant and are made up using approved backings and spacings they are laminated in three parts, backing, numbers and acrylic/plastic front.
They were a pain in the bum to do and i used to hate doing them as trying to get it laminated with no air bubbles very hard (well i thought so).
IIRC you could order in illegal number plates in store, as well as teh classic black ones but the store staff would/should advise you that they were illegal.