I've been updating my wardrobe over the last few months too and found
http://www.fashionbeans.com/ useful for fashion tips. I'm now buying clothes with a plan rather than continously buying indigo jeans and white Converse!
Places to check out:
ASOS - Good variety of clothes. Keep your eye open for discount codes.
T K Maxx - Always good if you're prepared to rummage, you can find some great stuff at low prices. Be prepared to pop into a store more than once before you come out with something you like.
Reiss - Wouldn't pay full price for much of their stuff but you can get good buys from the sales.
Massimo Dutti - Stylish and very reasonable prices.
H.E. Mango - Ditto above
All Saints - Sales are good though too much 'distressed' stuff for my liking
John Smedley Outlet (
http://www.johnsmedleyoutlet.com) - Quality knitwear - 50% off what you'd pay in places like Selfridges. If you wait until Fridays they reduce selected items. If you're near Matlock in Derbyshire the factory shop is worth a visit.
If you've got money to burn then check out
http://www.mrporter.com/. I've never bought anything off here but it's always nice to window shop and laugh at some of the stuff on there. Proof that money doesn't buy taste -
http://www.mrporter.com/product/385190