What kind of grades would I need to get into a top art university?

They want you to have a good general education but the important thing is your portfolio. GCSEs tend not to matter so much, but the only reason art unis list them as requirements is because there are a lot of different ways to get onto an art course, so it's their way of checking that you are smart enough in general. If you do A levels you will normally need to do a foundation course for another year after that too before you go to uni, or you can skip A levels and do an AVCE or something like that, so there are options. And to get onto any of those courses you need the right GCSEs, so if you complete that course then the uni will care about that more than your specific GCSE grades. And if you do A levels they won't look at GCSEs at all, so do your best but really don't worry about it.