[WillPS]

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

That'd make sense if the regional ancillary service disappeared during GMTV's hours, but they didn't (at least not in Central nor in Yorkshire by the sounds of things).

I suspect that either the whole thing was added locally when the news was, and each region did something to switch between presenting as their own service or GMTV Text + redressing page 600. Unless it would have been possible for GMTV to emit a single page ancillary service on Page 600 and for the region to add the rest of the range?