British Sign Language
From Tardis Wiki, the free Doctor Who reference
British Sign Language was a type of communication used between deaf people in the United Kingdom.
Cass, the second in command for an underwater mining facility in Scotland, communicated with the others only in British Sign Language, which was translated by fellow employee Lunn. The Twelfth Doctor attempted to respond to her in BSL but soon realised he had forgotten how to. (TV: Under the Lake)
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. Stone was also the first deaf actor to appear in Doctor Who.
