Winston Churchill had a habit of breakfasting in bed and remaining there – sometimes until as late as 1pm – with a secretary and typewriter near his bedside, according to rare papers and insights revealed for the first time by the Imperial War Museums.

Documents, testimonies and images showing what life was like in the Churchill war rooms – the former prime minister’s underground wartime HQ under central London – have been made public to mark the UK release of the film Darkest Hour, which recounts the early days of his premiership.

Among the collection is a rare surviving example of a letter confirming a typist’s appointment, as well as insights from Churchill’s private secretary, Elizabeth Layton, and other secretaries working in the underground bunker, including details of the leader’s working habits and routines.

“We of his personal staff were completely devoted to him, even though he was inclined to be impatient and demanding,” said Layton. “He was somebody who drew our loyalty and our deep respect and affection ... The negative side was only on the surface. Underneath he was a very caring person.”

Myra Nora Collyer, a secretary working in the war rooms, remembered the “masses and masses and masses of corridors” and the need for “sun ray lamps” once a week because they were working 12-hour shifts underground.

Margaret Walker, employed as a typist and secretary, said: “He used to wear a siren suit and we used to see this shortish, fattish, tubby man bouncing along in a siren suit.”

A siren suit, similar to a boiler suit, was an all-in-one garment which could easily be put on over other clothing and was used by people who had to leave their homes for air-raid shelters during bombing raids.

Other documents revealed by the Imperial War Museums include early plans for the basement headquarters.

The materials have been released to inspire people to visit the Churchill War Rooms in London, which are featured in Darkest Hour and allow visitors to walk in the footsteps of people who worked there.

Visitors will also be able to see locations which were recreated for the film, including the map room, the cabinet room, Churchill’s bedroom and the transatlantic telephone room.

Churchill is played by Gary Oldman in the upcoming film, which is set in May 1940 as Churchill, just days after becoming prime minister, has to decide whether to accept a negotiated peace treaty with Nazi Germany or fight on. It features three of his most famous speeches: his debut to parliament as prime minister, his first radio address to the nation and a speech to MPs following the Dunkirk evacuation.

Churchill’s premiership has become something of a cultural touchstone for politicians, both in the UK and across the Atlantic.

“In the year of Trump and the doubts a lot of Americans have about the whole concept of leadership, Churchill is about to get an extra boost,” Andrew Roberts, a British historian who has written a biography of Churchill, recently told the Guardian.

“It’s having a leader whom everyone admires and looks up to and is working towards a goal that everyone needs. It does remind people that kind of uncomplicated, unequivocal leadership can be out there.”

Michael Bishop, executive director of the International Churchill Society, wrote in an online review: “Gary Oldman inhabits the role of Churchill, capturing his wit, grit and determination and even his impish smile.

“Wreathed in cigar smoke, with a voice that ranges from a slurred mumble to a stentorian roar, Oldman looks and sounds the part.”