I believe the difference is like the FBI and CIA in the USA - one is for domestic intelligence and one is for international intelligence. I also think that some of the other numbers may have been active in intelligence and of the like during the world wars.MI (Military Intelligence) had agencies numbered up to 19, but not all at the same time.
NCIS is a team of federal law enforcement professionals dedicated to protecting the people, family, and assets of the US Navy and the Marine Corps worldwide. We do not have an agency called the National Criminal Investigative Service, because we have many federal agencies that investigate crimes nationally. MI5, the Security Service, is the UK's security intelligence agency responsible for protecting the UK, its interests and citizens against major threats to national security." However, these 2 distinct roles entail actual operational overlap and thus "the scope of national security extends beyond the British Isles and may involve the protection of British interests worldwide, e.g.The former is responsible to the Home Office and the latter to the Foreign Office.MI5 investigates matters of national security in the UK (investigates terrorists, counterinsurgency, etc).Equivalent to the US's CIA (Central Intelligence Agency) NCIS (national criminal investigation squad) are top ranking police officers dealing with high profile crimes, and have little to do with intelligence, though often co-operate with intelligence agencies for practical reasons. Anonymous' contention that NCIS stands for National Criminal Investigation Squad is a load of old tosh.NCIS is the National Criminal Intelligence Service, and far from being merely "top ranking police officers dealing with high profile crimes" it busies itself with identifying new criminal trends, acting as a clearing house for information from police forces around the UK, and liaising with Interpol, Europol, and various intelligence service around the world. Foot's SOE, The Special Operations Executive 1940 - 1946.GCHQ regularly recruit analysts, and have large teams who can understand and verify whether information is up to date, or translate documents and coded messages. Finally, all federal agencies have Offices of Inspectors General (OIG) who have special agents with law enforcement responsibility and authority to investigate fraud, waste, and abuse within and against each agency.