They might "self-certify" that their oil meets the spec.
If you can lay your hands on the original owners manual, it will probably say what oil the engine was designed to run on. It probably won't be the latest, most expensive, fully-synthetic that you can buy.
I have a Pug and the manual lists several oil specs according to the climate where it will be used. UK climate overlaps several of these so each would be acceptable. It also says the spec.
AFAIK, only Synthetics (annoyingly) have different specs for each motor manufacturer. I have another car that uses a Ford spec, and as long as you make a note of the spec number, you can match it practically anywhere.
If you look at the "choose your oil" page on sites like Castrol, it will encourage you to use the most expensive, but grudgingly also tell you which other versions meet the manufacturers spec.
Examples:
https://applications.castrol.com/oilselector/en_gb/c/search
or
https://www.halfords.com/motoring/engine-oils-and-fluids/engine-oil/?isVrnSearch=true#
I can use a 0/30 or an 0/40 or a 5/30 or a 5/40 for one car, and for the Pug, an 0/30, 5/40, 5/30, 5/40 or 10/40. When it was new, it recommended 10/40.
For me, Mobil 1 is the best value big-brand, but I can get a Valvoline change at National Tyres for a price which makes it not worth the trouble of changing it myself. I like to watch that they actually take the plug out rather than using a vacuum tube. Satisfy yourself that your local branch has a manager who runs a tight ship.
Frequent scheduled oil changes will do your car more good that using the most expensive brand.
Both my cars get changed at less than 10k mile intervals so the oil is always fresh and clear. If I had an old banger as well I could probably re-use it.