sorry, but reasonably priced and London do not go in one sentence.... haha, just j/k.
seriously, prices are steep, at least for major brands. I am guessing you are coming from the US? the exchange rate is also heavy but you can claim your VAT back, just don't forget to get the forms you need, i think either in a shop or the airport.
as for shops, check out the flagship TOpshop in Oxford STreet/Regent street. like chloe-babe said a bit further down there is Selfridges and that is also fun.
I would also go to Covent Garden and the surrounding streets - little boutiques and shops you can have a look at. have fun!