UK Guys, to truly experience Tex-Mex, you need to find a mom 'n' pop outfit preferably in West Texas, New Mexico, or Arizona, where the Mom half of the outfit is in the kitchen, using traditional spices and recipes. Heat is not the object, but there should be enough to catch your attention.
New Mexico, and El Paso, IMO is the mecca area for great mex.
I've tried "Mexican" food in places like New York... the result is more italian than Mex.
Haven't tried it in England. When I'm there, I eat (and love) huge platters of greasy pub food, and the Indian food is outstanding!