Most people who study art at Oxford have done a foundation course before hand. I would recommend not resitting year 12 if that’s what you’re thinking of doing, progress to year 13 and apply for an art foundation at UAL or somewhere similar so that your portfolio is on par with everyone else’s and so that you don’t have to pay for the foundation. I would resist any exams that you may not have preformed will enough on during that time and apply through UCAS.