Hi! Check out Christa Weil's books, Second Hand Chic and It's Vintage, Darling! She is a vintage clothing expert based in the UK (London, I think). Her books list several second hand stores. Second Hand Chic is especially informative. I've had my copy for almost 15 years and I refer to it often. She gives detailed information on brands/designers, fabric, shoes, accessories, etc -- a wealth of information!
I wouldn't count on finding vintage bargains in London! London has the most expensive vintage in the country, but try Portobello Market if you have time. I also have a friend who is a vintage stylist in London and may be able to help you. Go to www.vintagesecret.com and send her an email, she's very good and knows all the sellers and where you'd find the best deals.