There are a few little hidden shops I found the other day around the side streets of Carnaby St, there's an Adidas store that was doing alot of real hard to find kicks, with some hard to get clothing too. There was also a Levi store which again was selling some hard to find gear, then I found that Liberty sell Comme de Garcon if your ever after the more expensive stuff.
Ya dude the pounds gonna kill you, I spent a semester in central london last year, had a blast but ended up spending way too much money. Anyway, check the other threads, there's a little shop called Global Sports in some outdoor market ( i forget the name of the market but it's in zone 2 or 3 .... the details are in one of the other London threads). Global sports had a bunch of hard to find shit for somewhat reasonable prices (lots of air maxes! ). They even had the beautiful losers blazers for like 75 pounds even though they been sold out for like years (2 bad none in US size 11 )