Luardos food van do the best burritos and carnitas I've found and Wahaca is a good sit-down restaurant but expensive. (I keep meaning to get a copy of Mexican Food Made Simple by Wahaca's owner, Thomasina Miers.)
From Chile to Chocolate is a blog written by a man obsessed by Mexican food and is an invaluable resource for Londoners, with tips to help source Mexican ingredients plus ideas and inspiration. Mr Chile to Chocolate is currently in Mexico barbecuing cactus, so reading about his adventures provides even better inspiration. Drool warning. I need to order that book!