The Traffic Signs Regulations and General Directions 2016

8.  Where the sign is erected on a road subject to a speed limit greater than 20 mph and is within 50 metres of any lamp which forms part of a system of street-lighting, the illumination requirements for the sign are–—

(a) where that system of street-lighting is illuminated throughout the hours of darkness, the sign must be illuminated by internal or external lighting for so long as that system is illuminated and may also be reflectorised; or

(b) where that system of street-lighting is not illuminated throughout the hours of darkness—

(i) the sign must be illuminated by internal or external lighting for so long as that system is illuminated and must also be reflectorised; or

(ii) the sign must be illuminated throughout the hours of darkness by internal or external lighting and may also be reflectorised.