Sound advice above. One thing I would recommend is buying your clothes before leaving the US - it will do much less damage to your budget.
Pretty much anything goes here in London, depending on who you are, who you want to be and where you are. For example, as a 30 year old bloke I dress in a combo of Banana Republic, Gap, M&S (for basics) with a little Paul Smith and Nigel Hall knitwear thrown in.
The missus wears lots of Monsoon, a bit of Gap and M&S plus Jaeger for more formal wear. But pretty much anything goes. If you want to avoid look like an American tourst, a few gentle pointers
* Do not wear sneakers. Londoners love trainers, but they have to colourful, a bit 'street' and branded.
* Do not wear college baseball caps or tops
* Never, ever wear a fanny pack - or bum bag as they are know here.
But you probably already know that.
Fingers crossed for decent weather, and pray it doesn't get as hot as '03 when the thermometer hit 38C (>100F). Not pleasant in a city where a/c is still pretty rare.