The British Security Service (MI5) is the U.K.'s national security authority. It is responsible for protecting the U.K. against threats to national security. The MI5 website provides information on the current major threats to U.K. security and practical advice to help businesses and organizations to protect against them.
MI5 collects intelligence through "the interception of communications; eavesdropping, agents within target organisations; and surveillance. . . . MI5 holds around 20,000 active files, about one third of which relate to foreign nationals — typically members or associates of foreign intelligence services or terrorist groups — leaving approximately 13,000 active files on UK citizens."