My parents own thier own fashion company so I got a bit of knowledge on this one. Plus my sister is thinking of doing this (or designing) she's only at GCSE but has chosen Maths, English Lit, French and Fashion & Textiles for AS Level. Although I think she has chosen them as she enjoys them rather than because they are necessity.
I think you should definately try and get work exp a.s.a.p. with anything fashion related try getting a sales job at fashion shows perhaps (London, B'ham and Harrogate are probably your best bets), try and get as many contacts as possible along the way.
As for a degree there's no specific discipline to follow but I would guess Business Studies would be a good place to start. However I know someone who went to Harrods as a trainee straight after A-Levels completed a training course and was head-buyer of womens wear by the age of 21, he kept in touch with the Harrods people and now works for his parents company and makes loads by supplying his mates at Harrods.