Its no secret that they were almost certainly generated (or at least controlled) by BBC Micro computers (totally custom font, apparently), and YouTuber Tom Scott has been on the trail of looking for the source code for the clocks for some project for a while now, no joy.
However this video was posted which features footage of the time Grab a Grand went wrong once on the House Party (where the player literally grabbed it straight from Noel's hands!) and whoever was in control of the software for the clock ended up accidentally bringing the clock's control software to air revealing its sources:
(from 16:32 if it doesn't go straight there)
That led to this tweet:
I got an email from @elocemearg, who's made a full simulation of the early-90s BBC countdown clock! It's exactly the HD upscale I hoped for. Lots of options, easy to key or screen-record.— Tom Scott (@tomscott) August 16, 2020
The next time I need a countdown for a video, I'm using this.https://t.co/tOpZusrpSB
And we now have a 90s style BBC countdown clock generator if there was ever a need for one.
In March the BBC streamed a recording of a 1991 football match with that clock. The character edges weren't perfectly clean but sort-of good enough. I took some screenshots then and basically forgot I had them for five months until Tom's video a few days ago reminded me. pic.twitter.com/uQXy0TdxA2— Graeme Cole (@elocemearg) August 16, 2020
I get the impression looking at the follow-up YouTube comments that these may have have actually been generated by Archimedes but maybe controlled by a Micro? I don't know much about the BBC Micro hardware as to what it could do as I was an Amstrad kid at home but we had them at school (for funding reasons our school was stuck with them until 1996 when they all vanished and were replaced by Acorns!) I know they were used for the Domesday Project back in the 1980s, both running it and building the videodisks for it (indeed they were instrumental in extracting the content back out years later).