MI5, MI6. What happened to the other MI’s?

To combat and counter evil, James Bond is frequently sent to all corners of the world by his boss, Vice Admiral Sir Miles Messervy (otherwise known as 'M'), and they both work for MI6. The United Kingdom itself is protected by MI5.

If both MI5 and MI6 exist, the question arises whether the other Military Intelligence departments are still active. The Ministry of Defense (MoD) have never officially confirmed or denied the existence of these departments.

A sort of comprehensive list that  mentions the potential codes for these elusive departments can be found on wikipedia. It puts forward a 'tentative list', comprising of:
- MI1: Distribution of reports, intelligence records - Interception and cryptanalysis - The Secret Service/SIS - Communications security - Wireless telegraphy - Personnel and finance - Security, deception and counter intelligence.
- MI2: Originally responsible for geographic information on Middle and Far East, Scandinavia, the Americas (excluding Canada), Spain, Portugal, Italy, Liberia, Tangier, the Balkans, the Ottoman Empire, Trans-Caucasus, Arabia, Sinai, Abyssinia, North Africa (excluding French and Spanish possessions), Egypt, and the Sudan. After World War I its role has been changed to handle Russian and Scandinavian intelligence. These functions were absorbed into MI3 in 1941.
- MI3: Originally responsible for geographic information on The Netherlands, Belgium, Luxembourg, France, Morocco, Austria-Hungary, Switzerland, Germany, Norway, Sweden and Denmark. Also responsible for Military translations. After World War I, its role was changed to intelligence in Europe, after 1941 including the Baltic states, USSR and Scandinavia
- MI4: Originally responsible for responsible for aerial reconnaissance, interpretation and the removal of landmines. Operations of MI4 officially ceased in September 1947
- MI5: Domestic Counterintelligence and Security
- MI6: Foreign Intelligence and Security
- MI7: Originally responsible for responsible for censorship - foreign and domestic propaganda, including press releases concerning army matters - translation and (from 1917) regulation of foreign visitors - foreign press propaganda and review. Now defunct.
- MI8: Originally responsible for Interception and Interpretation of Communications
- MI9: Originally responsible for aiding resistance fighters in enemy occupied territory, recovering (escape and evasion) Allied troops who found themselves behind enemy lines, debriefing of escaped British PoWs, interrogation of enemy PoWs
- MI10: Originally responsible for weapons and technical analysis during World War II.
- MI11: Originally responsible for protecting British troops against the enemy agents amongst civilian population during and in the theatre of war.
- MI12: Liaison with censorship organisations in Ministry of Information, military censorship.
- MI13: Never used.
- MI14: Originally responsible for Germany and German-occupied territories (aerial photography until Spring 1943). Originally part of MI3.
- MI15: Aerial photography. In the Spring of 1943, aerial photography moved to the Air Ministry and MI15 became air defence intelligence.
- MI16: Scientific Intelligence (established in 1945)
- MI17: Secretariat for Director of Military Intelligence from April 1943.
- MI18: Never used.
- MI19: Enemy prisoner of war interrogation and debriefing (formed from MI9 in December 1941).
- MI20 - MI25 remain classified (or have never existed at all).

Most of this information is still shrouded in mystery. Some MI departments might, for instance, have merged or still exist alongside the ones we know about. Others might not have existed at all.