https://www.allstarcard.co.uk/this-matters-fuel/fleet-news/supermarket-fuel/
This link pretty much agrees with everything said. Petrol is petrol. No matter where you buy it from. However they add additives in branded. Even the basic ones. So not just v power. Do these additives make a difference? No one knows as the evidence so far has been inconclusive. So some says yes, some says no.
It's like is mineral water or tap water better for you IMO.