Another exception is that the rules came into effect in September 2001. Plates fitted before that do NOT need to show maker/postcode.

Also, I don't think it's an MOT fail.

I have 2 pre-2001 cars, one has the original plates which DEFINITELY have no postcode.
The other has newer Halfords plates (with GB symbol for foreign travel), you have to look VERY HARD from specific angles to see the supplier details, they're effectively invisible to a casual glance.