The intelligence community of the United Kingdom is both older and more complicated than that of the United States. MI5, or the Security Service, and MI6, the Secret Intelligence Service, are the most well known components of the British intelligence structure, but these are just two parts of a vast intelligence apparatus. Communications intelligence is the responsibility of the Government Communications Headquarters (GCHQ), which works closely with the Communications Electronics Security Group, while a number of agencies manage military intelligence under the aegis of the Ministry of Defense. London's Metropolitan Police, or Scotland Yard, has its own Special Branch concerned with intelligence.
The "MI" by which the two principal British security services are known (MI5, or Security Service, and MI6, or Secret Intelligence Service) refers to their common origins in military intelligence. Both can trace their roots to the Secret Service Bureau, created in 1909 after a report by Parliament's Committee on Imperial Defense concluded that "an extensive system of German espionage exists in this country..." Working with the War Office, Admiralty, and various operatives and agents overseas, the bureau had both a Home Section and a Foreign Section--precursors, respectively, of MI5 and MI6.
Command and control operates through no less than four entities: the Central Intelligence Machinery, the Ministerial Committee on the Intelligence Services, the Permanent Secretaries' Committee on the Intelligence Services, and the Joint Intelligence Committee. (more)