In the UK, Mexican food is equated with the "throw some more curry in, luv, I like 'em hot!" crowd and we are increasingly finding chillis added to everything, fairly blindly.

That's definitely not the case here in the US. Some Mexican restaurants in the US may be like that, but there are just as many that are not.

The vegetable fajitas I like to order (when available) are just grilled vegetables (usually squash, onions, tomatoes, mushrooms, and bell peppers), with flour tortillas to wrap them in. You can put salsa on them if you want, but that's purely optional. I like to put some of the refritos (refried beans) and rice that comes with the fajitas in the tortilla with the veggies, and top it off with some guacamole. There's no chilis or anything else that's hot in this combination unless I add salsa.