The vast majority of the lacto-ovo veg meals I've had over the last year in C to and from Asia have been dire. UA also assumes that ordering a vegetarian meal must mean you are some kind of health freak, so you don't get the normal dessert. Instead they give you some crappy fruit or sawdust cookies when all you really wanted was the normal piece of cheesecake like everybody else.
Coming from LHR, Indian (or Hindu or whatever they are calling it) is definitely the way to go. You'll get actual Indian food (maybe even cooked by an Indian or Pakistani).
I used to always order Asian Veg or Hindu, but on US originating flights it always seemed to be substituted with a Vegan meal, so I quit. All the Vegan meals I've had on UA have been terrible. It's usually something like sauteed red pepper strips over white rice or sauteed mixed vegetables. The kind of thing your Aunt Mildred might dream up if you made her cook you a vegetarian meal. I'd love to send a few cookbooks to these catering people.
Asian vegetarian in C should be easy- some vegetarian sushi, miso soup, yaki tofu, maybe some noodles, rice. How hard is that?
Surprisingly, I don't think I've ever had a good meal out of BKK, on any carrier, on any route, in any class.