MI6

MI6, aka SIS (Secret Intelligence Service), was a branch of the British government concerned with foreign intelligence. Like its domestic counterpart MI5, it was sometimes simply called the Secret Service. Kim Philby explained to the Eighth Doctor that despite the fact the two organisations were theoretically working on the same side in two separate spheres, MI6 and MI5 frequently clashed. (PROSE: Endgame) On another occasion, the Doctor told Leighton Woodrow he had believed that MI6 had no jurisdiction within Britain's borders, a belief that Woodrow found refreshingly naive. (COMIC: The Fallen)

MI6 had a history of alien encounters (COMIC: The Fallen, The Flood) which led to the formation of a division of MI6 to combat alien threats, Wonderland. (COMIC: Hunters of the Burning Stone, The Clockwise War, et al.) However, C claimed to Team TARDIS that MI6 discredited the existence of extra-terrestrial life as late as 2020, leaving the matter to UNIT and the Torchwood Institute. (TV: Spyfall)

History
In January 1933, the First Doctor and Susan encountered an SIS agent named Pollitt in Berlin. (AUDIO: The Alchemists)

In Vienna in 1948, the Sixth Doctor posed as a SIS agent named Dr. John Smith. (AUDIO: Quicksilver)

In 1963, an MI6 agent called Barker secretly worked with Russian spies, in particular a man who went by the alias of Miles Barrington Smythe. On 23 November, Miles left secret documents for Barker which instructed him to use his government employee card to access information in William Pike's cab before being arrested by the Police. Barker questioned Bill Pike, who was also arrested, although he was interrupted by K9 Mark 2, Jorjie Turner and Starkey. K9 was able to prove Bill's innocence by scanning the document and finding the instructions involving the use of a government employee card, Bill was let free as he wasn't a government agent. K9 almost exposed Barker although the document self-destructed before he could find his name. Barker and Miles later met up where Barker gave him a spare copy of the document. (TV: The Cambridge Spy)

In the 1970s, the Secret Intelligence Service was one of the agencies which had its funding reduced by the British government. (PROSE: Where the Heart Is)

By 1997, MI6 was employing double-O agents. (PROSE: The Dying Days)

In the early 21st century, MI6 sponsored a genetics lab in which Grace Holloway worked on creating a human-Time Lord hybrid capable of regeneration. (COMIC: The Fallen)

In 2005, (COMIC: Hunters of the Burning Stone) under Director Leighton Woodrow, MI6 became aware of an alien incursion in London, but deliberately did not mention it to UNIT and tried to handle the situation themselves; this inadvertently led to the agency being massacred by Cybermen and an unprepared Earth being assaulted. (COMIC: The Flood)

Right after this incident it was decided that MI6 needed to expand their borders to outer space using the alien technology, using the codename Wonderland. They used the Eleventh Doctor's sonic screwdriver to adapt the Cyber-Technology and retrieved an Amaranian Star-hopper to be their ship. (COMIC: Hunters of the Burning Stone)

In October 2009, after the destruction of the Torchwood Hub, the director of MI6 met with other department heads to discuss how to handle the situation. MI6 and MI5 were exploring the possible buyers for the alien technology gathered from the Hub. (PROSE: Long Time Dead)

Around 2020, C was the head of MI6 until he was assassinated in his office at the headquarters in London. (TV: Spyfall)