Our family vehicles run almost exclusively on Morrisons standard grade fuel. Petrol engined Astra 1.6, - Jazz 1.4, - Panda 1.2, - Punto 1.4 8v. Diesel engined SEAT Cordoba 1.9 tdi VE, - Fabia 1.9 tdi PD, - and our newest addition Kia Rio 1.4 tdi. No (fuel related) problems with any of them. Broadly speaking I don't hold with aftermarket additives, - especially oil additives. Fuel additives? undecided, but I ran Wynns Eco Diesel in the Cordoba because of it's additional lubricating qualities, because now we have low sulphur diesel, to protect the high pressure pump and injectors. Never managed to convince my boy to run it in the Fabia though. I recently replaced the Cordoba with a direct injection petrol Ibiza and, as I'm concerned about the possibility of carbon build up in the inlets (a direct consequence of it being direct injection) I run Archoil AR 6900-P MAX in it. The Archoil also provides increased lubricity which I'm hoping will help protect the high pressure pump and injectors. A few years ago our venerable 1992 Panda Parade (Felicity) developed a blow in her head gasket - No 1 cylinder. I was most impressed to see, once I'd removed the head, how clean the inlets were (Single point injector). Even within the cylinders themselves and the head, there was no excessively great carbon build up. We had owned her since she was just a few years old and ran her on Morrison's fuel maybe 99% of the time. All of the vehicles get very regular oil changes with top quality, OE manufacturers spec, oil.
I used to, socially, "bump into" a tanker driver a few years ago. He told me that pretty much all the fuel in our area comes out of the Granton refinary, and, as someone above has said, the additive packages for the different brands get added depending on who has purchased that individual load. Of course I can only speak from my own experience but I think you can look at this from various points of view.
1) Unless you are a "performance" driver with a car that can take advantage of the higher octane fuel, just use standard supermarket fuel. The additive package is unlikely to be the best but, based on my experiences, it would seem to be adequate. It's worth considering too that supermarkets have a very high turn over so you are never likely to encounter "stale" product.
2) Premium grade fuels would seem to have performance advantages if you have an engine and ECU that can take advantage of it. It's very probably that there are other beneficial elements too but the whole thing is shrouded in secrecy and I find it impossible to find out anything definitive. Some People report advantages others seem to notice no measurable effect.
3) Because I have no problem with supermarket fuels (and Mrs J collects the "loyalty thing") I decided, many years ago to just run it. However where I can spot a particular area of concern (ie the lubricity deficit due to removing sulphur from Diesel and the, much reported, inlet carbon fouling on modern direct injection petrol engines) I will research it and look for solutions - hence the Wynns Eco Diesel for the Cordoba and the Archoil AR 6900-P MAX for the Ibiza. I really don't know for sure but I think addressing specific areas of weakness in this way, rather than the "scatter gun" approach of using the premium grade product, just may be more effective?