For UK, it looks for a special mark being used by ITV companies (and Channel 4, which are ultimately controlled by IBA) which is mainly used to signal companies in other regions that they should switch to an advertisement tailored for their region [1]. Since the television channels were controlled by IBA and BBC, this is a for-sure way to skip advertisements.

1: https://en.wikipedia.org/wiki/Cue_mark#Cue_dots_in_televisio...

I think those VCRs looked for the short black screen between the programming and commercials

I thought it was triggered by audio volume, because ads are typically louder and more consistently loud. Maybe it was a combination. Not so sure how well it ever worked, though.

I remember from my childhood seeing an icon in the screen corner indicating adverts were about to start. This was on terrestrial British TV.

Frasier had long enough black transitions between scenes to trigger the fast forward on mine