In Knightsbridge there is Pandora's Box which is a rather large designer second hand store. Though, it is a second hand shop, the items the store carries are suprisingly select and all quite new or unworn. Last seasons "it" Cavalli dress or "have to have" Miu Miu jackets can be found there.
But don't expect it to be cheap. A £2000 dress is about £1300 at Pandora's, as its possibly only been worn once.
Theres also a smaller store next to it that stocks bags and more shoes, and a rather large store on Hampsted highstreet.
If you want vintage wear there is also the unmissable world renowned "One of a Kind" in Portobello. Sadly for mere earthlings like that it is appintment only, as Naomi Campbell and Kate Moss are known to trotter around, but if you want the less extravagant version (so not vintage peal embroidered Valentino but Vintage LV speedys and lovely lace dresses) head for One of Kind Too.