I don't think it is anything to do with cartels but regulations which limits the amount of advertising that can be shown in an hour. These adverts should be spread equally throughout the hour, not in one big block or ten small blocks.
Broadcasters prefer their programmes to start on the hour or half hour so the 'rhythm' of adverts will be much the same across all commercial stations.
I first noticed the 'coming up' and recap before and after each ad break in a C4 renovation programme some years ago - could have a Sarah Beeny series.