10 Downing Street is the official residence and the office of the British Prime Minister. The office helps the Prime Minister to establish and deliver the government’s overall strategy and policy priorities, and to communicate the government’s policies to Parliament, the public and international audiences.
According to Film London: 'Darkest Hour was one of a rare few fiction feature films to be granted shooting permission outside 10 Downing Street, an iconic location which has had to be fabricated in-studio by past productions.' The production team were not allowed to make physical changes to the location so the specific visuals had to be altered in post-production to reflect how it looked in 1940.
“I doubt whether we would have got permission to shoot directly out the front of Number 10, or have Gary Oldman coming out through the front door, had the subject matter not been about Churchill,” says Adam Richards, the film’s supervising location manager.
(source The Knowledge)
A thrilling and inspiring true story begins on the eve of World War II as, within days of becoming Prime Minister of Great Britain, Winston Churchill must face one of his most turbulent and defining trials: exploring a negotiated peace treaty with Nazi Germany, or standing firm to fight for the ideals, liberty and freedom of a nation. As the unstoppable Nazi forces roll across Western Europe and the threat of invasion is imminent, and with an unprepared public, a skeptical King, and his own party plotting against him, Churchill must withstand his darkest hour, rally a nation, and attempt to change the course of world history.