I think everything's basically already been said, but I just wanted to say -
where I used board, you put your laundry out and it was taken and washed and returned. However, people used to make the mistake of thinking it would come back immediately. Well, it often took a while, and the people who put all their laundry in at once would have no clothes for ages!
So, take as many shirts and underwear and stuff as is physically possible. Not only does it mean you'll always have some clean, but it also helps in a worse case scenario (three of your shirts spontaneously set alight, someone borrows something without asking, the laundry's back very, very late...etc, etc). Primark, New Look, and any supermarket will have tons of shirts in different colours, and they'll be fairly cheap.
Our school's theoretically got a 'smart' dress code for sixth-formers, but most of the girls just wear any dark/checked/striped skirt they have with a jacket and t-shirt/polo shirt/blouse. For the rest of the time, take whatever you're comfortable in. It really depends on the school how often you'll have formal-ish events, but you probably won't need much in the way of 'dressy' clothes.
And I know you said not to say it...but Jack Wills and Abercrombie always go down well (:
Ask if the school has its own shop, or has one on campus, or that you're allowed to go without needing to beg for permission. My school has a little village shop down the road that has literally everything (including toiletries etc), but there's no ATM for about 1000000 miles, so make sure you've always got cash if you think you might need to buy stuff. Also, some people at my school (I don't board, but some people do) have things like kettles in their rooms. No idea if you'll be allowed one, but it's the sort of thing you'll want to sort out with your room-mate.
Have fun! Boarding really is awesome, although you'll appreciate going home, sleeping in a non-cramped bed and using a shower with decent water pressure!