Mucho Mas is ok... good for London. It is still not as good as the states and they don't use real sour cream, and they have fake cheese ( or at least it tastes fake because it has no flavor ) and they are always out of chicken and guac when I go...those are my two faves, so that is annoying.
A friend of mine ( who is also obsessed with Burritos ) took me to a place down the street called Tortilla last week. A better burrito for sure. Still not as good as the places in the Mission in San Francisco...but pretty close! The best in London I think.