Don't shop at BP.
The stores are such a rip off. The want 69p for a pint of milk that costs 45p everywhere else. If it were a small independent retailer I wouldn't mind but they're not. I also don't care if they try to wash their hands of any kind of price setting because of the shop franchises. All I know is when I see their sign I know to keep driving. Other filling stations are no better.
My advice is think ahead. There is nothing convenient about their stores.