If you're in London and you're dying for a Mexican fix there are some places to recommend. Don't miss these! (I'm a SoCal, so I know my Mexican food!
Mestizo -- the best authentic Mexican food in London IMHO, just up from Warren Street tube. GREAT tacos and mains, fantastic bar. The cochinita pibil (Yucatan-style shredded pork tacos) is to die for. Lovely staff and service. They have a small cupboard of things for sale as well -- moles, nopales, sauces, etc. and canned goods under the La Costena label (sort of like Mexican Heinz, good quality, in every Mexican household's pantry).
Taqueria -- in Notting Hill, end of Westbourne Grove Road. They usually have a full cupboard of Cool Chile spices and sauces, tortillas, etc ... and it's a regular restaurant, Mexican-run, that does an upscale version of Mexican street grub, so it means you don't have to wait for Friday or Saturday to go to Borough Market. Very organic, you can have a great meal there and take away lots of goodies with you.
El Panzon -- a Mexican restaurant in a pub in Brixton owned and operated by an Irish chef and cooked by Mexicans. Lovingly prepared, huge portions, great value for money! Just around the corner from Brixton tube.
Green and Red -- in Shoreditch, in Bethal Green Red. Pricey but authentic "resort-style" Mexican food. I've only eaten there once cuz it's expensive but the barbecued king prawns Vera Cruz style were legendary.
I've tried 'em all ... hope this helps!