IMHO the best Mexican in London is at
Mestizo. Wahaca is good and nicely fresh, but not really standard Mexican fare. And Green & Red
used to be good, but they just had a change of ownership and their chef left (he told me to open up his own place, squeeeeee!). So Mestizo it is.
There's an awful lot of "tex-mex" in London, which is just huge amounts of stodgy, greasy meat and cheese, which I don't think is very good at all. I judge a good Mexican by 1) do they make their own tortillas? and b) how good is their molé? Not by how much cheddar cheese they throw on their burritos.