BP unleaded low MPG

[Dylanfan]

I agree, less additives in supermarket fuels, diesel in particular. Also where I live, no supermarkets sell super.

[TonyHSD]

Well, people are saying all petrol stations supermarkets or independent, large chains or small businesses should conform to the BS and they are correct. The thing is that this can not always be monitored in real life conditions. 
There are some places that will do this on paper only , some unknowingly may sell lower quality due to the dirty and contaminated tanks etc.  
All fuels in UK comes from a few  well known refineries I get that, however their products goes through few middle men before they get into our cars, the transport company, the filling station. The storage tanks also play a role. There are no two fuels the same in your tank even bought from garages that belong to the same company and are located on the same road a mile between each other. Sometimes even you can get small differences in fuel bought from the same place but at different days, different batch , slightly different additives at the time.
The idea is to try few different garages, find your best performance/ efficiency fuel and buy from there.  
For me the best fuel is from local BP with M&S shop, it’s not cheap but has the most consistent quality and my car likes it the most. 
I can sense the difference especially in how the engine is running and stick with them whenever is possible. I also fill up from other places from time to time and the difference always comes up and it’s felt immediately as more noise, sluggish acceleration, engine rough idle, vibrations etc.