In a Delivery Document / Broadcaster guidance, if this is relevant then specify what language tag should be used. It is suggested that the BBC asks for bfi-x-makaton. bfi is registered as "British Sign Language". x-makaton is a custom / private addition to indicate that this is a special kind of this sign language.
The suggestion is:
bfi-x-makaton
.bfi
is registered as "British Sign Language".x-makaton
is a custom / private addition to indicate that this is a special kind of this sign language.