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.
   http://www.mi5.gov.uk/

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.
   http://www.gchq.gov.uk/

National Criminal Intelligence Service  
   The National Criminal Intelligence Service (NCIS) is at the forefront of the fight against serious and organised crime in the UK and throughout the world.
   http://www.ncis.co.uk/

National Intelligence Machinery  
   Report details the roles, structure and funding of intelligence services. It also contains details of the Cabinet Office contribution to the co-ordination of intelligence efforts.
   http://www.official-documents.co.uk/document/caboff/nim/natint.htm

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.
   http://www.surveillancecommissioners.gov.uk/

Foreign & 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.
   http://www.fco.gov.uk/servlet/Front?pagename=OpenMarket/Xcelerate/ShowPage&c=Page&cid=1059736061019

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.
   http://en.wikipedia.org/wiki/Secret_Intelligence_Service

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.
   http://en.wikipedia.org/wiki/MI5

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.
   http://www.met.police.uk/so/special_branch.htm

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. [PDF]
   http://www.scotland.gov.uk/library5/justice/sbwuk.pdf

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