I recognise someone in that video who I used to work with - I knew he was at Thames but not that he'd been in the control room as it went off air! Fascinating clip. For the more technically minded among us, would the TC have had to put up the off air audio to hear the Carlton intro, surely they wouldn't be listening to that normally
It might have been a routine thing for off air confidence for the normal Thames to LWT switch-overs. Remember
often in the late 80s early 90s the 17:15:00 switchover would start with Thames News (via LWT's pres). Thames' MCR would have still liked to make 100% sure the audio was getting through OK. Then later; programmes (Home and Away ?) would straddle that switch. Again if Thames were playing out, no better way to know it's leaving them, is to hear and see it coming back from Crystal P.
TVS to Meridian didn't take the ITN bulletin, only Big Ben itself, and kept the same shot, but the sound faded out just before midnight - of course there was no move for the tx output there as far as I know.
To this day I don't know whether that sound fade out just before midnight was malicious or accidental.
You can just see a slight glitch about 10 secs in, I was watching at the time, and that's the point the Teletext Ltd data inserter was switched into the video path.