1) Beyond Retro -
a safe option if you're new to london, there are two one located in central london, one just off oxford street (it says just off oxford street but it is a fair walk) or the easier, brick lane on Cheshire Street the closest tube station being bethnal green. The prices have been getting steeper and steeper however i find that if you go there when they have a sale on (generally beginning of summer or new year they drop quite a lot!
2) Traid -
this is a second hand store that donates all profits to charity, you can find anything from old vintage st. michael's jumpers to Armarni shirts for as little as £10!!! my favourite is the one on camden high street, and fortunately there is one located right on my local high street in woodgreen! there are 11 shops around london so if your a local just google it!!
Now i realise that this is a high street store but i just CANNOT get enough of it! and now its available ONLINE!!! yes from the comfort of your own bed! however the one zara shop i would recommend would have to be the one in knightsbridge, needless to say they have everything there! but if your looking for friendly staff or people who can help you with your purchases this is NOT the zara to go to! in fact i dont think there is one in london i have yet found in london that offers the type of service i would expect! if you want service try blue water!
4)Charity Shops -
The best charity shops are most definitely in Marylebone, i am yet to check these out myself but i have been ensured by many others that if you want vintage designer for reasonable -yet still fairly steep- prices then you have hit the jack-pot!
5) Vintroville -
This is a shop i LOVE and cannot express how much everyone should check it out! it is the coolest new shop around! its located on hackney road right next to a well known recording studio, and yes they quite often have welcome surprises in the form of Florence Welch, The Kooks, Kelis and before they had the shop they had a market stall which Michael Corrs visited more than once! even if you dont spot any celebs you're sure to pick up a vintage bargin! http://vintroville.com/