Government Communications Headquarters
Includes details of current job vacancies and a quiz for potential applicants.
http://www.gchq.gov.uk

MI5 The Security Service
The Security Service (commonly called MI5) carries out security intelligence work aimed at combating threats to the nation such as espionage and terrorism.
http://www.mi5.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

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