[Stuart]

Was that caption made on a BBC Micro? The font looks like one Teletext would've used.

I think they were produced by the transmitters themselves after the end of any signal from the broadcaster.  I doubt they looked for anythng particularly complex as a way of creating them.

IIRC that is in similar style to the one during the ITV strike in the late 1970s, which was well before any BBC Micro existed.