" . . . no person shall–
(a)use, or cause or permit to be used, on a road any vehicle which is in motion–
(i)between sunset and sunrise, or
(ii)in seriously reduced visibility between sunrise and sunset; or
(b)allow to remain at rest, or cause or permit to be allowed to remain at rest, on a road any vehicle between sunset and sunrise
unless every front position lamp, rear position lamp, rear registration plate lamp, side marker lamp and end-outline marker lamp with which the vehicle is required by these Regulations to be fitted is kept lit and unobscured"
- Section 24, the Road Vehicles Lighting Regulations 1989.
The use of the word 'every' seems to make it clear that the use of lights on just one side of the vehicle does not meet the legal requirements.