I have a friend who had a tiny, tiny waist, and she always used to only really be able to get things at either Next or I think River Island, for the same reason.
Can't be 100% certain, because she modified a lot of her clothes for dancing, etc, but I think they had the ones that were the best for her for sure. I remember going with her to Peacocks once as well, their petite section was pretty good for her. But there aren't as many Peacocks around these days.