Well, there was the infamous Shell Optimax affair but it was a few years back now (Optimax was the precursor of V-Power).
Any issue with fuel sold at supermarket forecourts is usually one of contamination, either during transport or storage, and not an issue with the quality of the fuel per-se.
I think we can agree that it's unlikely that only supermarkets suffer from fuel contamination issues, so I would hazard a guess that the reason they are in the news is due to the high volume of sales, meaning that a contaminated underground storage tank will affect very many cars in a short space if time and wreck very many engines.
Ordinary filling station will have much lower footfall in comparison, and a contamination issue at the pumps will affect a smaller number of cars before it's detected, and therefore unlikely to make the headlines. Just a guess.