MI5? MI6? Who?
How the UK's foreign intelligence service got its name.

The Secret Intelligence Service is also called MI6. It originated in 1909 as the Foreign Section of the Secret Service Bureau.

It was responsible for gathering intelligence overseas. The SIS title became official in 1922. Then the busy days of WW2 started. The Special Operations Executive (SOE) was established in 1940, partly from the then Section D of SIS. After the War it was disbanded. Some of its members were re-absorbed into SIS.

See the excellent history 'A History of the British Secret Service' by Richard Deacon (ISBN 0-586-05116-3) for details. The Intelligence Services Act placed SIS was placed the Foreign & Commonwealth Secretary, which explains Burnside’s continual interaction with Wellingham.

Since this service operates abroad, it is ultimately to this ministry to which it is responsible. This act defines the functions of the Service and the responsibilities of its Chief, as well as establishing control and oversight arrangements.

The Service's primary role is the production of secret intelligence supporting British security, defence, foreign and economic policies within requirements of the JIC and approved by government.

As the CIA is known as 'The Company'. Reportedly, and only reportedly, SIS is known internally as 'The Firm' and by other agencies as 'The Friends'. This may explain a line of dialogue when Willie is in Malta when a MI5 officer identifies himself as ‘a friend’.