I have to say that most of the "proper" Thai food I have had has had a fair amount of sauce, and rarely particularly thick sauce. I used to work with someone from Malaysia and we'd regularly go to a nearby Thai restaurant that she proclaimed was very authentic and they were certainly saucy.
I have also seen Thai which almost always has less sauce, which is much thicker - kind of like curry sauce in a way. More heavy on the coconut. And I've generally assumed that was more Anglicised. Don't recall ever having it in a Thai restaurant, funnily enough.
I've never been to Thailand. I don't know how authentic any UK version of Thai is. I just know I've had Thai food I felt was naff, and some that was excellent and have personally rated Marple Thai Kitchen are up there in my mind. Compared to the gloopiness of most local Chinese takeaways, the food feels refreshing and tasty.