Length That Follows the Story
Line up British dramas and you notice that the lengths are all over the place. A season of Sherlock is three 90-minute episodes. Doctor Who began on 23 November 1963, reached 694 episodes by 1989, stopped there, and came back in 2005. There is a Guinness World Record too — though the current title is not "longest-running science-fiction series" but "the science-fiction TV series with the most episodes" (819 episodes as of 29 April 2017).
Black Mirror tells a completely different story each time, and moved from Channel 4 to Netflix in 2016. Its creator, Charlie Brooker, has said he read a biography of Rod Serling of The Twilight Zone (→ The SF Anthology) and was drawn to how Serling tried to tell difficult subjects in the shape of a fable. He considered borrowing that device of a narrator appearing at the top of every episode, and decided against it — because when people remember The Twilight Zone, what they remember is not the narrator but the show itself.
You often hear the explanation that British dramas are short because of public-service broadcasting. This map has not yet come across institutional documents to support that, so it is not written here. What could be confirmed is the other thing: that the lengths differ from work to work.