To get onto a university foundation course, you'll probably need a minimum of 5 GCSE's at grades A to C (preferably including English, Maths and Art) plus at least one A-level in a relevant subject (e.g. Art or Design). This course would probably last for one year (possibly two). You would then be eligible to progress to a 3-year university course which, these days, is almost the only route into fashion design. See here:
http://www.learndirect-advice.co.uk/helpwithyourcareer/j obprofiles/profiles/profile596/
Chris