The British Secret Service was the governmental body responsible for intelligence gathering. Two of its branches were MI5 and MI6.
The Seventh Doctor forged letters of authority from the Prime Minister and the Head of the British Secret Service to gain access to the ULTIMA project. (TV: The Curse of Fenric)
Malachi Yarrow headed a section of the Secret Service that dealt with Mal'akh in the late 20th and early 21st centuries. (PROSE: Subjective Interlock)
The Service covered up the massacre of the Mars Probe 13 astronauts, spreading disinformation that Mars was uninhabitable and sabotaged NASA data in order to discourage further missions. (PROSE: The Dying Days)
Colonel Peter Rathbone was a South African member of the British Secret Service. He was tasked with getting any secrets that the Triskele had. (AUDIO: Storm Warning)
The United Kingdom kept open the four World War II "citadel" bunkers in London, so the British government could continue operating in a nuclear war. The Service secretly built a fifth. (AUDIO: The Fifth Citadel)