Halfords plate have the head office postcode reflectivly etched onto the background... I know this as I worked for them when it was all rolled out
Ive never had an MOT fail because of the Postcode not being on there, plus they have to prove that you got them made up after the legislation came in (otherwise everyone in the UK would have to change their plates!) So cant see them kicking up a fuss about it. For as long as they know, that persons owned that personalised plate for years with those acrylic plates