[Bluecortina]

My understanding is that at the regional centres their presentation area was switched out of line during GMTV and the signal path went through their news gallery.

If so my guess would be that the teletext for each company was added before that switch and GMTVs passed through the gallery. Therefore the teletext service broadcast just followed which was on air.

The main Teletext Ltd service didn't care which company was on air as it was added right at the last point of the TX chain

Pre 1993 national teletext was bridged across from the incoming network circuits to the outgoing circuits with additional lines added if required by the inserters, plus ITS. Teletext/ITS etc would be stripped off before being fed to the internal distribution within a tv centre. Post 1993 it would be the same but obviously just the local teletext service including subtitling etc.