I guess the sbtitles were hardcoded in the signal, right?
That would seem to be the case. I believe there was at least one Spanish-language broadcast with subtitles burned in as well.
While the Holiday Special was not aired in England as far as we know, it was apparently aired in the Republic of Ireland’s RTÉ 2 on Christmas Day of 1979…so that broadcast would have been PAL.
–SKot