I read once years back (feel free too correct this anyone, it's only what I read and might be BS), that all fuels basically come from the same 5 or 6 UK refineries and are the same thing nationwide, refineries don't produce twenty different grades of fuel for different customers it would be too costly, but It's the additives and cleaning agents added by/for the retailer that affect it..
Supermarkets tend to put more in, up to 5-10%, to dilute it and get more volume, so they can sell it cheaper as they have more volume.
Other garages like shell, BP etc, put less additives in, around 4-5%, and their premium fuels have less again like 1-2%, so they cost more but is less diluted so gives better mpg and performance...
Only thing that's ever swayed me, few years back went on family holiday to bottom of Cornwall.
Filled up at tesco before leaving, think was around £40 at the time. Just about got there, almost running on fumes at arrival.
Coming back, I filled up at shell as was only one around, cost few quid more, but got home with it just under 1/4 tank left.. Still had another 40-50 miles in it..
Got much better mpg from the shell..
Also my mate got a mega tuned 2.0 turbo, it's running nearly 400hp..
Got a computer in that adjusts boost etc, gives a load of power settings etc on screen, and he swears his setup tells him he gets more bhp when using stuff like shell optimax or wherever it's called that basic or budget fuels..