You will need the permission of the Police and obey control measures if you intend to use certain special effects, fake firearms, weapons, police uniforms, marked Police vehicles etc. Where appropriate, especially when special effects and pyrotechnics are to be used, local residents and businesses should be informed in advance. All effects should be overseen by a qualified experienced Supervisor.
The Health and Safety Executive provides comprehensive online information sheets:
Special or visual effects involving explosives or pyrotechnics used in film and television productions.
www.hse.gov.uk/pubns/etis16.pdf
Stunts, fights and other potentially hazardous production activities.
www.hse.gov.uk/pubns/etis17.pdf
Management of firearms and other weapons in productions
www.hse.gov.uk/pubns/etis20.pdf
Filming of scenes involving nudity, offensive language, etc. must be shielded from the public and where appropriate should be approved by the local authority and Police.