The specifications of petrol and diesel are tightly controlled but this only applies to the octane and cetane rating respectively , additives are up to the seller and are added to the raw fuel at the bulk fuel depots. There is almost no tolerance on the stated octane rating for petrol but the cetane rating for diesel is a little wider. Both can be sampled at the pumps by the customs and excise organisation without notice to check on compliance with the rules.
My son who runs a diesel is convinced that he gets better performance when not using supermarket diesel which in his case is usually Morrisons.
You buy what you can afford and/or what you are happy with for your vehicle. My Jazz is mostly run on BP 95 octane, I gave up on our local Tesco where I bought their Momentum 99 when they went over to card only payment at the pump during the Covid 19 crisis. If you wanted a paper or something from the shop you could go in there and pay in cash having been forced to use a card at the pump, no logic.
Vic.