British Sign Language

British Sign Language, or BSL, was a visual form of communication used by, and for the benefit of, deaf people in the United Kingdom.

In 2119, Cass, the second in command on the Drum, an underwater mining facility in Scotland, communicated with her team only in British Sign Language, which was translated by fellow employee Tim Lunn. The Twelfth Doctor attempted to respond to her in BSL, but soon realised he had forgotten how to, apparently having deleted it in favour of semaphore. (TV: Under the Lake)

An intelligent suit taking care of Fern knew sign language, but knew this was futile as Fern was dead, and would never move again. (PROSE: A Rose by Any Other Name)

Behind the scenes
British Sign Language was used on-screen by a character for the first time by Sophie Stone, who played Cass in Under the Lake and Before the Flood.

Zaqi Ismail, who played her translator Tim Lunn, knew sign language because his sister was deaf. The two had to create several new words in sign language for more science fiction and story-specific terms.

One Doctor Who fan, Elizabeth Harvey, age 14, wrote to Doctor Who Magazine about her delight in seeing BSL and deaf signers represented in Doctor Who, in a letter featured in DWM 493. According to her, though, a deaf person should always have two interpreters accompanying them, in case of sickness, or, in this instance, "death followed by ghostliness".

In 2016, a video of the Twelfth Doctor signing "Merry Christmas and a fantastic New Year!" was released on Twitter.