The postcode side of it doesn't come into the mot all the tester has to check is the sizing and font if they are correct it is a pass.
No plates are a grey area there has been lots of changes to the legislation so it does get confusing.
Anyone supplying road plates needs to be registered and correctly stamp plates.
Hence why you get 'show plates' they are not legal for road use.
All new plates bought now must have all markings.

Not that my cossie has any of these other than the rear plate being correct size!