Hey there :]
Topman would definitely be the most likely place to find a range of sizes, especially the flagship store in Oxford Circus, London. If you don't have much time, you can order online at topman.com :]
As Guy said, American Apparel have a good range of sizes, and their products are really good quality. Again, they have a store in London (Carnaby St.) and an online store at store.americanapparel.co.uk :]
River Island sometimes has XS sizes too, but not in all of their lines, so it's a bit hit and miss.
H&M occasionally has smaller-sized tops, and you're quite likely to find smaller-waisted jeans and trousers there, all for a good price.
Gee, I sound like a sales clerk :]
Hope this helps xx