I haven't been here for awhile so maybe this has already been discussed... but I found a great shop in Covent Garden - Cybercandy at 11 Shelton Street. Lots and lots of USA candy, gum, cookies, lifesavers, oreos! .... Prices are ridiculous but definitely worth a visit. Check it out at cybercandy.co.uk
Highly recommended!
I left with Jolly Rancher Watermelon candy, Bubbleyum and Twizzlers. But will go back for more next time I'm in London.
Enjoy!