Unlike subtitling, I'm unaware of any TV provider (e.g. Sky, BBC, etc) that offers signing as an option. If you're seeing it, it must be part of the actual broadcast, that everyone sees.
Slightly off topic, but there was a channel I watched a few times, and I could not get rid of the " descriptive video". I did a little digging and found out that the channel broadcast with that feature built-in, and I couldn't disable it...I don't watch in anymore.