Unlike most British people, I've had quite a lot of exposure to Mexican food, as I have visited Arizona frequently over the years due to having family there. Unfortunately, Britain has not generally been good for Mexican food but that's slowly been changing over the years.
I've sampled a few Mexican restaurants in London so I thought I'd do a quick round up of them. Short version: go to Lupita. It is awesome.
I've been to the Wahaca on Wardour Street a few times. It has some interesting vegetarian options, I especially like their vegetarian main course salad, and the fact that they experiment with interesting combinations, such as lancashire cheese and black bean. They put the same degree of creativity into their vegetarian options as the rest of the menu which is really nice. They also have a really interesting drinks menu - various types of tequila are perhaps to be expected but they also have some Mexican non-alcoholic drinks that are interesting and a bit different to what you would normally get at a Mexican restaurant.
They lose points for me for other things though: atmosphere - the place is incredibly noisy and not all that comfortable); and the fact that they bring the food to you at random intervals, which they claim is about the authentic street food experience (um, wouldn't that involve an actual street?) but seems to me to be more about laziness about co-ordinating things in the kitchen, I love tapas/mezze style eating but not when it's forced on me and I don't get any choice about what dish comes when.
Because it's geographically quite handy for me, I've also been to Tortilla on Southwark Street a few times. It's a nice place to go if you're after a relatively cheap and cheerful meal in the area - two of you can have a burrito each and a pitcher of (very tasty) margaritas for around £20 which is pretty good value in London.
I went to Mestizo for Sunday lunch a few months ago as they do an all you can eat lunch, with as much wine and beer as you like, and that seemed like a good idea. It wasn't. The food was ok - though the vegetarian offerings were more minimal than the website suggested - but the main issue was that the service was pretty surly, we had the strong impression that they resented us for not being Mexican and we got significantly worse service than anyone else there.
I've been to La Perla on Charlotte Street a couple of times. The vegetarian options aren't terribly exciting but they are well executed and it has a tequila flight which is nice for those of us who haven't had a chance to try much nice tequila. It's a pretty nice all rounder, the only reason why I wouldn't go there more often is because it's not Lupita and Lupita is only a short walk away.
On that note, Lupita, on Villiers Street is, hands down, my favourite Mexican restaurant in London. The food is delicious, with many and varied interesting vegetarian options. I'm a particular fan of the "Chicharron de Queso" which is essentially a thin piece of deep fried cheese. Nothing wrong with that! Last time I went with a vegetarian friend and we had the vegetarian platter for two which, at £18.50, is quite expensive but gives you an extraordinary amount of very nice food. The margaritas are also very very good. The only real downside to Lupita is that the atmosphere isn't great - it's crammed full of tables and is quite noisy. The downstairs is a bit quieter but still not great. Given the food, though, I don't care!