You don't. And, for similar reasoning you don't know the more expensive fuel is "better" quality.

More importantly you know that pretty much any fuel being sold in a mainstream place in the UK is going to meet the minimum national standards which are perfectly fine for the vast majority of cars on the road.

Anyone that has a car that requires E5 rather than E10, or higher octane fuel may need to buy the associated "premium" fuels, but these are just not necessary for the vast majority of cars on the road. But we're not talking about the premium fuels here, we're talking about two garages selling pretty much the same thing for quite different prices and preying on some people's FUD.