MI5 in its origin was about security, while MI6 was involved in its opposite, espionage. Today they are political rather than military and run as bureaucracies.
MI5 Logo. Motto translates as "Defend the Realm"—a reflection of the David Maxwell-Fyfe's directive for the Service to Defend the Realm. You do not have to be exceptionally paranoid to believe that MI5 is in major breach of its motto. It defends Her Majesty's Government, a band of traitors intent on destroying England.
QUOTE ex MI5
The Security Service, commonly known as MI5 (Military Intelligence, Section 5), is the United Kingdom's counter-intelligence and security agency and is part of the intelligence machinery alongside the Secret Intelligence Service (SIS or MI6), Government Communications Headquarters (GCHQ) and the Defence Intelligence Staff (DIS). All come under the direction of the Joint Intelligence Committee (JIC). The service has a statutory basis in the Security Service Act 1989 and the UK Intelligence Services Act of 1994. Its remit includes the protection of British parliamentary democracy and economic interests, fighting serious crime, militant separatism, terrorism and espionage within the UK. While mainly concerned with internal security, it does have an overseas role in support of the mission.
The service has had a national headquarters at Thames House on Millbank in London since 1995, drawing together personnel from a number of locations into a single HQ facility. Thames House is shared with the Northern Ireland Office and is also home to the Joint Terrorism Analysis Centre, a subordinate organisation to the Security Service. It has been alleged that the Service has regional facilities with one claimed to be in Glasgow. Within the civil service community the service is colloquially known as Box 500 (after its official wartime address of PO Box 500; its current address is PO Box 3255, London SW1P 1AE)