trust me physics would come in handy, maybe do it up to as level because A2 physics is not as related towards engineering but as unit one does have quite a bit that is important to engineering. physics isn't needed for petroleum eng, but it would be good to have it, it will make you a better candidate through the ucas process , just make sure you get the grade in chem and maths, also doing further maths will help you , especially the mechanics and further pure modules.