The Public Service Vehicles (Conduct of Drivers, Inspectors, Conductors and Passengers) Regulations 1990
"4.—(1) A driver shall not, when a vehicle is in motion, hold a microphone or any attachment thereto unless it is necessary for him, either in an emergency or on grounds of safety, to speak into the microphone.
(2) Subject to paragraph (3), a driver shall not, when a vehicle is in motion, speak to any person either directly or by means of a microphone.
(3) Nothing in paragraph (2) shall prevent–
(a) the driver of a vehicle from–
(i) speaking in circumstances when he is obliged to do so by reason of an emergency or on grounds of safety; or
(ii) speaking to a relevant person in relation to the operation of the vehicle provided that he can do so without being distracted from his driving of the vehicle; and
(b) the driver of a vehicle which is being used to provide a relevant service from making short statements from time to time limited to indicating the location of the vehicle or operational matters provided that he can do so without being distracted from his driving of the vehicle."