221 Lights liable to endanger
(1) A person must not exhibit in the United Kingdom any light which:
(a) by reason of its glare is liable to endanger aircraft taking off from or landing at an aerodrome; or
(b) by reason of its liability to be mistaken for an aeronautical ground light is liable to endanger aircraft.
I feel this is more to cover lights other than aerodrome lighting such that users displaying such lights can be legally required to extinguish them:-
(2) If any light which appears to the CAA to be a light described in paragraph (1) is exhibited, the CAA may direct the person who is the occupier of the place where the light is exhibited or who has charge of the light, to take such steps within a reasonable time as are specified in the direction:
(a) to extinguish or screen the light; and
(b) to prevent in the future the exhibition of any other light which may similarly endanger aircraft.
If it's a "private" airfield there is no statutory requirement to have any lighting at all for "private" flights subject the provisos above. Duty of care and insurance are other matters which may apply.