Best deals/selection online, otherwise, Ultralove, Love's Touch for overpriced cheap costumes & don't forget to check out the normal costume shops that are cashing in on adult fetish attire these days:
Bizarre Novelties on 2nd and for post labour day/halloween shopping season go to Dressew on Hastings, Value Village & family friendly London Drugs.
Second hand stores are a great resource as well (and carbon/karma good), easy & inexpensive to assemble a schoolgirl costume. Bohemia Gallery on Main between 16th & 17th always seems to have sequins & hooker heels in the window & the kooky owner is forever in giddy deal making mode. Never met such a perennially happy shopkeeper!