In researching the Snowdrop stamp for an earlier entry, I discovered that the UK General Post Office at one time had its own film unit. GPO Film Unit was set up in 1933 and was primarily (but not exclusively) responsible for making documenting GPO-related activities. This unit was run by Scottish documentary filmmaker John Grierson. More on Grierson can be found at the University of Stirling's Special Collections in the John Grierson Collection.
Rolls of cine 35mm film & cans. Photograph. Encyclopædia Britannica ImageQuest. Accessed Oct 27, 2021.
The unit was under the auspices of the GPO until 1940 when it was moved under the Ministry of Information and renamed the Crown Film Unit.
The British Film Institute has some of the films available to watch for free. They also released a collection of the GPO films in 2008 under the title Addressing the Nation. Some of the films are also available on YouTube. One of my favorites is Rainbow Dance (1936) directed by Len Lye. It's a film ahead of its time mixing positive and negative images, animation, color, along with modern dance and music. The film actually was an advertisement for the GPO Savings Bank!
Additional reading on the GPO Unit and its films: The
projection of Britain: a history of the GPO Film Unit / edited by Scott Anthony
and James G. Mansell. New York: Palgrave Macmillan, 2011. 9781844573745
Final Fun fact: Grierson is credited with coining the term "documentary" in his anonymous New York Sun review of the film Moana directed by Robert Flaherty (Nanook of the North fame). Grierson signed his review as "The Moviegoer." You can read the review in its entirety in the second edition of Lewis Jacob's The Documentary Tradition, New York: W.W. Norton, 1979.