Can't get CC subtitles working

I have a new LG TV which supports subtitles and I can see subtitles if I watch a programme directly through my TV.

I also have an Apple TV box. I would like the Apple TV remote to be the one remote I use, but I CANNOT figure out how to do this (ie get subtitles working).

I have repeatedly gone into settings using the Apple Remote and set subtitles either "Auto" or "CC" and setting the language but nothing works. Sometimes when I have tried and failed to get subtitles working, I find the app (in this case BBC iPlayer) has somehow changed Subtitles to "Off". On other occasions I go back into Settings to find the subtitle language has been changed to unknown.

I'd appreciate suggestions on how to get this working.