Commonwealth Office - Secret Intelligence Service (SIS) - Describes MI6, a Crown Service responsible for obtaining secret information and conducting operations in support of the UK’s foreign policy objectives, and to counter threats to UK interests worldwide.

Cabinet Office - Intelligence and Security Committee - Offers news, list of committee members, and links to annual reports and other official publications.
Meta Description: [ Intelligence and Security Committee reports and information. ]

GCHQ - Official site of the UK Government Communications Headquarters which is the centre for Her Majesty's Government's Signal Intelligence (SIGINT) activities. Includes details of current job vacancies and an online quiz for potential applicants.

Investigatory Powers Tribunal (IPT) - Official independent tribunal investigating complaints against the Intelligence Services, Police and public authorities relating to interception, surveillance and other conduct.

Metropolitan Police Service - Special Branch - Provides a brief history and a list of responsibilities, including intelligence-gathering and operations on extremist political and terrorist activity.
Meta Description: [ Metropolitan Police Service - Special Branch ]

MI5 The Security Service - Provides information on the current major threats to UK security and expert security advice designed to help businesses and organisations protect against them.
Meta Description: [ This is the official website of the Security Service, commonly known as MI5. The Service is responsible for protecting the UK against threats to national security. ]

National Criminal Intelligence Service - Information on the work of the organisation charged with fighting serious and organised crime.
Meta Description: [ NCIS, National Criminal Intelligence Service. To provide leadership and excellence in criminal intelligence to combat serious and organised crime ]

National Intelligence Machinery - Official document explaining how British security and intelligence agencies are organised and funded.

Office of Surveillance Commissioners - Provides information about the roles and responsibilities of those who authorise and conduct covert surveillance operations and covert human intelligence sources.
Meta Description: [ Home page of the Office of the Surveillance Commissioners ]

500 Scottish Executive - Guidelines on Special Branch Work in the United Kingdom - Official report describes the roles and functions of this intelligence and policing service at national, regional and local levels.

Secret Intelligence Service - Official site of this government department also known as MI6. Features background information, contact details and career opportunities.

Wikipedia - MI5 - Hyperlinked encyclopedia entry provides the history of the Security Service, responsible for protecting British Parliamentary democracy and economic interests, and fighting serious crime.

Wikipedia - Secret Intelligence Service - Hyperlinked encyclopedia article about MI6 (Her Majesty's Secret Service) provides a history of its role beginning with the First World War. With list of directors.