i used to work for warehouse and they are quite a trendy place, put more formal than say H&M or topshop. i cant remember what i wore to the interview, but i think it was along the lines of black trousers and some groovy shoes (not your usual black stilettoes but something a bit different if youve got it) and a nice top, i dont think you are going to get any brownie points for going in a suit. show you like fashion and that your interested in it.
also, i remember they asked questions in my interview about the latest trends so swat up on that before you go, so you look like you know your stuff about whats hot at the moment. if you check out handbag.com theres loads of info about trends etc.
its a great place to work and the staff discount is a great bonus, only problem is that id spent my wage before id even earned it!
good luck
x