Just been reading some of the London reviews and have consistently been coming across the "there is no good Mexican or Japanese food in London" comment.
As a New Yorker (NYC) splitting my time between London (80%) and NYC (20%) over the last 3 years I would agree to avoid 99% of them. However, I have to point all of you in London who are craving good Japanese at a decent London price and Mexican to the following spots below. If you're visiting London please go for what you can't get at home.
Inaho Hereford Road W2
Hands down this is the best Japanese restaurant I have come across in London and holds its own with many of favorites in NYC and meals I have had in Tokyo and Osaka. Its a tiny spot (16 to 18 people max - they take reservations) and is very reminiscent of the tiny restaurants you come across in Japan. My wife and I eat there almost once a week and average 25 GBP less alcohol per person for a starter and decent amount of fish. The yellow tail is always good and the agadashe (spelling) tofu is to die for and I have never had a better salmon skin roll in my life. Beware the clueless Korean waitress, one of two working here over the past few months. Also, when ordering be sure to specify the number of pieces (sushi, sashimi, and maki) you want as there is endless confusion.
Otherwise its consistently a very good Japanese meal in London and I'll probably regret promoting it as it can already be hard to secure a table.
On to Mexican.
The Cool Chile co. (see link) use to run a stall on Portobello and one in Borough market. The have recently closed them to open a restaurant in Notting Hill where Mendola use to be. The tacos they were selling at their stalls were great and I expect the same from the soon to open restaurant. Can't vouch for the restaurant in Hampstead they reference on their web site. Though will definitely check it out and report back.
In the interim I'm getting my Mexican fix at Conran's little spot - Crazy Homies (horrible name) on Westbourne Park Rd. You can actually get a good Margarita and the salsa and tacos are decent to good. Though don't tell Eileen from the Portobello stall that I've been sleeping with the enemy (actually Conran and the Cool Chile co are on good terms).
I know all of this is very Notting Hill focused. However, I do get around London a lot and still have not found anything that fixes my cravings like the above.