It's all down to the Octane rating. In any case all the fuel around here comes from Stanlow.........Esso, Texaco, Shell, you name them I see their tankers queued up at the terminal every time I pass. TBH, there's very little difference to the fuel made at any refinery in The U.K, they all use the same blend of Crude Oil and the specifications that gasoline is made to are very tight. Fuel additives are also made to a tight spec. Back in the days when leaded petrol was the norm I used to do octane ratings when I was on nights and cetane ratings on diesel too, it was industry practice to round the numbers up so 98.6 would be rounded up to 99. Not much left the plant that was actually 99.
Pete