One theory is that it was done in the style of a BBC soft-opt.
TV-AM put out the first caption, BT Birmingham send that to Central as well as to the TX. Central genlock to that and put it on their output. BT switch Central to the TX, they fade to black and then bring up their own slide scanner and music.
It's possible, though it's rather a lot of effort to go to, for something the few viewers watching would have paid any attention to !
I wonder if standard procedure was for TV-am to play out the caption until 09:19:55 then fade to black, and for the ITV regions to output black until 09:20:05 then fade up their version of the same caption ?
In regions where BT were able to perform electronic switches (almost certainly the case for Birmingham) they'd perform the switch at 09:20:00 ? Doesn't explain why there's no visible splat in that video, but perhaps that day TV-am and Central were by chance more or less in sync ?!