I have been asked to reply.
The fitting and use of sirens is controlled by the Road Vehicles (Construction and Use) Regulations 1986. Under regulation 37 (5) the fitting of sirens is restricted to vehicles used by specific emergency and law enforcement agencies; including the police, fire brigades and ambulance service.
Regulation 99 (5) provides that such sirens may only be used either to indicate to other road users the urgency of the purposes for which the vehicle is being used, or to warn them of the presence of the vehicle on the road. Training and guidance on when it is appropriate to use sirens for these purposes is a matter for the individual agencies. We have not issued any guidance to the police as this is an operational matter for individual chief officers.