I'm yet another U.S. expat who misses good Mexican food and have just discovered that Angels & Gypsies in Camberwell does really nice tacos, but they're only available at lunch during the weekdays. They make their own soft corn tortillas, and I'm currently obsessed with two of their specials, the sea trout (in a lime and ginger sauce, I think), and the blackened shrimp. They also offer cochinita pibil, carnitas, carne asada and I think one featuring chorizo. All of the ones I've tried have been good, although the seafood ones were particularly outstanding.
The weekday lunch menu, which isn't posted on their website, also includes a few interesting salads and a selection from their normal Spanish tapas menu. The tapas is also excellent, by the way, and the dining room has a lovely atmosphere so I'd recommend going at other times as well, but I just wanted to spread the word on the tacos, since they don't seem to publicize them and I think they're worth trying.