That showing isn't subtitled, I admit I'd have expected the non-Wales showings to have been.
It probably would have been quite a difficult task back then, unless they had some way of keying on Ceefax 888 in real time?
These showings were primarily intended for Welsh speaking audiences who for whatever reason couldn't receive BBC Wales, as I understand it.
In-vision subtitling was possible ISTR - the BBC had special subtitles cap gens with a very distinctive typeface (grey background) which were used on a lot of foreign movies and language learning TV series in the 80s and early 90s. News also had a system to do live subtitling of News Afternoon (which had in-vision subtitles for the hard of hearing in the days before CEEFAX sets were widespread)
Whether it was possible, or cost effective, in the time available I guess is the question. However showing Pobl y Cwm in England, Scotland and Northern Ireland without burned-in subtitles seems a somewhat odd decision... (*)
Did S4C run their Transl888 service back then (English subtitles on 888, Welsh-for-hard-of-hearing subtitles on 889) - or were we in the pre-888 era? (And did S4C provide English subtitles for their Welsh-language content back then?)
(*) Interesting 'fun fact'. In Norway they tend not to burn in subtitles for non-Norwegian language shows and instead broadcast them as optional subtitles, so if you speak good English, you can turn them off for English language shows etc. Sweden tried this, but there was too much confusion amongst audiences, so they returned to burning in.
HOWEVER - subtitles are a real issue for visually impaired viewers if they don't speak the foreign language, as they can't read the subtitles (which is why BBC editorial policy is for dubbing rather than subtitles in most cases, outside entire shows and movies, and why News usually voice over foreign content in news reports).
In Sweden they have a solution. The subtitles are burned in live on broadcast (not on the masters) and the same text data is fed to a voice synthesiser (and a very good one) which can then read the subtitles on an optional audio feed (a bit like we have for audio description) so you can watch a foreign show and have the Swedish subtitles read to you.