Er, I already answered this in a newsgroup (uk tech digital tv?).
Basically, most people get subtitles direct from the MPEG2 datastream via proprietry software (in
sky digibox or Freeview box). The teletext system also exists separately - with subtitles on it, page 888 in UK - but is not "primary delivery mode", hence may not be as good (not "designed for real time", so there might well be delays).
At the moment, BBCtv via Astra2 just transmits Ceefax page 888 - for teletext only - and nothing else! On ITV, apart from page 888, I'm not sure whether the "text" pages are actually in teletext format.
Anyway, there's not much you can do about it!
For info, Google: "mpeg2 transport stream".
(Added), hang on though, surely you've got a "subtitles" option, which should fetch the MPEG2 subtitles directly (just like a
sky digibox!)? Does this not work?
In fact, I've just got the 9100 instructions off Humax website. Section 4.3 is setting subtitles and language, so the facility should be there!
Also see section 9.2