Special lights on vehicles used for fire brigade, etc., purposes
11.—(1)  A lamp showing a light of such colour as may be approved by the Registrar may be carried on a motor vehicle used for fire brigade, ambulance or police purposes or a vehicle owned by a police officer or a fire service officer or a civil defence officer and used by him in the execution of his duties.
(2)  A lamp carried in pursuance of this rule shall be fixed to the roof of the vehicle, and on or as near as practicable to the longitudinal axis of the vehicle.
(3)  The light shown by a lamp carried in pursuance of this rule shall be in the form of beams rotating in a substantially horizontal plane.
(4)  No vehicle shall, in pursuance of this rule, carry a lamp showing a light except when the vehicle is being used for any of the purposes specified in paragraph (1) and it is necessary or desirable to do so either to indicate to other road users the urgency of the purposes for which the vehicle is being used, or to warn other road users of the presence of the vehicle on the road.