Uni courses are all about UCAS points earned through AS, A levels, BTEC's etc.. (basically any formal study post GCSE). If you left school without going on to further education (ie; college), you wont have the required UCAS points to qualify for entry on to a degree level course. If this is the case, you would need to apply for either a BTEC diploma or similar and complete that before being able to move onto a degree course. However, if you have been in education since GCSE and have the UCAS points, then there's no reason why you cant get onto a degree course as millions of students take a gap year between A levels and degree. It's more likely that the courses you're applying for are over subscribed and so the colleges are looking to take those with recent, relevant educational experience (ie; qualifications in art and/or design).
Try arranging a meeting with the head of department at your first choice college/uni to discuss your options and how best to go about getting onto your chosen course. take a portfolio of art/design if you have one.