This newspaper advert mentions seeing ABBA on Disco on 12 October (1975).

Disco was a music quiz show hosted by Terry Wogan and was broadcast on Sunday afternoons on the BBC (presumably BBC1). Apparently it only ran for a few weeks.

ABBA may have appeared on the panel or as video clip. Unless anyone has a particular memory of this, I would assume that it was the video clip that was shown (or perhaps the Top of the Pops performance was repeated. If so, and the BBC still has this Disco programme, that would be awesome!).

NOTE: This programme is completely different to the German programme of the same name.

Thanks to Michael Price, William Wilson and Joszef.
Thanks to David Broomhall for programme information.