To add to the comment by Mr Morse
Judging by the fact that all the distributors tend to share storage depots ( eg statements on Hemel Hempstead) across the country and pipelines, there is little or no difference in what they all have for a particular grade.
There are some additives, most of which are developed and supplied by 3rd party companies such as Lubrizol.
Based on this there is no benefit, other than a higher RON rating.
Im trying out Optimax which is higher rated, to see if the quoted additional miles are true or not. If not Ill go back to standard 95.
As for cheap petrol, Sainsburys is often beaten by BP who also do the nectar points. Another reason they are losing to Tesco.