Standard petrol and diesel is all the same, bar the additives that different brands will add claiming extra mpg and cleaning etc and of course you will pay a little more for a brand than you will say tesco in the same way you pay more for heinz beans than Tesco own. Super unleaded and super diesels are a step up and thats why you pay the extra but trust me, all petrol and diesel before additives are made the same exacting British standards.
Given modern cars and the ECU and sensors that run every single aspect of the engine, do you honestly think we wouldn't have hundreds of thousands of cars broke down every day if supermarket fuel was watered down?