+1 on what stew said, this is government issue, profit made on fuel especially at supermarkets is nominal they do it to get you in and spend money at the store. you top up, and while your at it you go get some extras at the same time.
i can't remember where i read it, but i'm sure i read somewhere that one of the fuel companies made more money on sandwiches and snacks than they did on fuel.
i hate to say this but the french have the right idea having tolls on the motorways. but to do that we'd need to scrap road tax. then maybe i wouldn't mind paying so much for my fuel.