Physics is actually considered more important than Computing for many Computer Science courses. I would advise you to drop the Graphics and continue with Maths, Physics and Computing, resitting a Computing exam or some coursework to try and bring your grade up to a B by the end of A levels. However, if you absolutely detest Physics and don't think you can continue with it, email the uninversities you're interested in and ask if they would accept you with just AS Physics.