4.7.1. Presence, condition and operation
You must inspect all motorcycles fitted with front and rear position lamps.
The rear registration plate must be lit. Most motorcycles will light the registration plate with a rear position lamp, but some will use a separate registration plate lamp.
Some registration plate lamps may be fitted behind the number plate.
Some mopeds might not have a registration plate lamp.
Defect Category
(a) A rear registration plate lamp throwing direct white light to the rear Minor
(b) A rear registration plate lamp or light source missing or inoperative:
(i) when rear registration plate has 2 or more lamps or light sources
(ii) when rear registration plate has only one lamp or all lamps not working
Minor
Major
(c) A registration plate lamp:
(i) not securely attached
(ii) likely to become detached
Minor
Major
https://www.gov.uk/guidance/mot-inspection-manual-for-motorcycles/4-lamps-reflectors-and-electrical-equipment