Unfortunately not all channels and apps available have subtitles.
OFCOM regulates and monitors most of the broadcasters and on demand providers , although that process generally consists on retrospective six monthly reports on whether they have achieved their targets on subtitling.
BT TV packages and their add on give you access to a range of channels provided by different broadcasters and on demand players that are the responsibility of their respective owners. so while there is some good subtitling there are some significant gaps. For example the NOW Tv app on the Youview platform that the existing BT Tv boxes use does not yet provide subtitles although the live BT multicast delivered recordable channels do. Several of the Terrestrial Aerial channels that are broadcast over Freeview do not provide subtitling.
Your BT TV subscription for NOW Tv packages would allow you to use NOW TV app on another device (eg your TV ) to view your subscribed on demand content for subtitles.
Whether the new BT TV box Pro launched this week will improve this situation remains unclear.