HELP!!!!!! Yank with sticker shock [London]
Try the various places in Brixton Village. See the mega thread here: http://chowhound.chow.com/topics/698606
Close to the Kensington area, this might be useful: http://chowhound.chow.com/topics/762910
Less pricey places mentioned in that thread include: No. 10, Mohsen, Daquise, Bosphorous Kebab House
You're very close to Taiwan Village and that whole high street (North Rd?) has a lot of cheap eats as well as a market. There's a fried to order falafel stand which I think is 3 pounds for an enormous "large."
Otherwise check the other budget threads. Depending on how far you want to travel on the District Line, check various Indian threads regarding Whitechapel and East Ham.