As per below would be illegal but I had some fitted to my LCR and passed 3 MOTs with no issue with them fitted. The only time it was mentioned when one of them was out. Getting a pull for it seems extremely petty though.
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This inspection applies to all vehicles fitted with front and rear position lamps.
Registration Plate Lamps
Registration plate lamps are the lamps that illuminate the rear registration plate. Some vehicles may have these lamps fitted behind the number plate.
Multiple Registration Plate Lamps
Where more than one rear registration plate lamp, or bulb, is fitted as original equipment, any one inoperative lamp or bulb is a Reason for Rejection.
Tricycle and quadricycle requirements are detailed at Section 9.1. C. Registration Plate Lamps
Check Operation
1. Turn on the position lamps and check that registration plate lamp(s):
a. are fitted
b. are secure
c. are working
d. are correctly positioned
e. does not show white light to the rear.
Reason for failure
1. A rear registration plate lamp:
a. not fitted
b. insecure
c. with any bulb not working
d. not illuminating the rear registration plate
e. shows white light directly to the rear