This is an article from last year but I’m sure many of the shops mentioned are still there. I like how the distinguish the different types of food.
Generally, if you are looking for a specific type of food e.g. Gujarati, then head to an area that has many Gujaratis. Wikipedia supplies this breakdown of where each different group are mostly found.
If you are looking for Gujarati vegetarian sweets and snacks (like chevda, ghatia etc) then I wholeheartedly recommend Gayatri which is in Kingsbury – be prepared to queue on festival days. Oh damn, now I’m hungry and missing London :S