[LRS]

I seem to remember that some years ago there were some good articles on the ‘actual’ added additive contents and their percentages that fuel retailers provide e.g. Shell, Esso…) The supermarkets are harder to confirm as I believe they can buy direct from the refinery or from one of the main fuel retailers e.g. Shell, or even a mix.

I seem to remember that what comes out of the fuel refinery is generic – plain old BS unleaded petrol or diesel.

However, once each fuel retailer receives their generic fuel from the refinery, they then add in their own specific cocktail of additives at their own specific percentages. (Refinery's can also do this on behalf of the retailer at source)

So, for the most part “petrol is petrol” but you then have the individual additive cocktails and percentages of each fuel retailer to think about.
You are most likely getting a better product – based on the additives and percentages, from one of the main fuel retailers e.g. Esso, BP, Shell, rather than from the supermarket retailers.