BP have no refineries in Britain anymore. They use their buying power and long term contract to purchase product at a good rate. If they do not need their quota they can sell it on at a small profit and still make money. As long as the volume consumed reamins constant they are ok.
As has been said its the government that makes the big money, they benefit from duty on petrol and also increased insome from the high price of north sea oil.
Pressure on the government is the only way forward,