Yeah its the main reason why i go to tesco too. And that's a shame you have to to dismount also don't make a hugely common mistake that Shell's fuel is different to Tescos. It all comes from the same refinery. But with that said, All the bikes i've owned seem to prefer tesco high octane additives over stuff like v-power etc... i feel like the have more "umph" or punch on tescos. But yeah, the only differences in the fuels are the additives.
Re petrols being the same from the same refinery etc, absolutely true. There's a BS-EN number for conformity of fluid, this sets minimum standards and tolerances which also tie up with type approval and Euro emissions compliance.
What can be altered, to an extent are additives. That's it.
There was some article recently that sight weather over a year you'd be better off in overall running costs of using branded over supermarket. I would suspect you would need to be doing a huge mileage to realise that.
I'm a believer in if it works for you, or you trust your proven source, then stick with it unless you have good reason not to.
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