If you're lucky enough to be heading to London anytime in the near future, this is where I recommend you go:
1. The markets: absolutely the best way to shop over here. You can literally find anything, (including this really classy nurse uniform, as elegantly modeled by my flatmate in Camden) and its guaranteed to be unique and generally not too expensive. My favorite is Portebello Road [pictured above]; it has a great atmosphere and inspiring aesthetic. The cheapest (and craziest) is Camden Market--a good place to pick up souvenirs for friends. East London markets are a little more expensive, but spend a Sunday browsing Spitafields, Sunday Up, and Brick Lane markets and you're sure to find something.
2. The vintage stores: Londoners have great style, meaning their consignment stores are chock-a-block full of great stuff. East London has the highest concentration of vintage shops--Blondie, Absolute Vintage, and Rockit are the major ones. Pretty much every neighborhood has a range of second hand stores, stocked with everything from designer scores to stained tees.
3. The boutiques: I read through travel books, magazines, and websites and made a long long list of boutiques I wanted to visit while I was here. Honestly, I haven't seen half of them and don't really mind. It's impossible to see (or locate) all of them. The best thing to do in London? Take the tube somewhere, get out, and walk around. Fashion cardio. Wander in and and out of any store that catches your eye. Covent Garden, Notting Hill, and east London all have a dense population of cute places.
4. The chains: The greatest thing about European chains is that they carry trends long before their US counterparts. It's easy to find really cool stuff for really cheap prices if you shop around enough. My favorite inexpensive chains are Primark, Dorothy Perkins, Next, Topshop, H+M (I swear it carries completely different stuff than the American stores. Way, way better.), Zara, Office, Joy, and Uniqlo. So basically, every chain. I never want to shop at the King of Prussia mall again.
Regardless of how much you actually buy, wandering around the shopping areas in London are overwhelmingly inspiring. (at least the less-touristy areas. Do not, I repeat, DO NOT head anywhere near Oxford Circus on the weekends. You'll have an anxiety attack and regret it.) You'll get loads of style ideas from the store displays and other shoppers. So much better than flipping through a magazine or catalog. Though the UK has a reputation for bad customer service, I've found the people working in stores and market stalls to be incredibly friendly. However, this could just be because their intrigued by American accents.
As for my purchases, this is what I've gotten (not counting lots of souvenirs and such for other people): two dresses (a simple grey shift and a feistier leopard number), two belts (a white elastic one and a jaguar one), tortoiseshell sunglasses, charms, and a very gigantic black sun hat (that's the black thing looming in the corner.)