Since she was English, I suspect Christie would have considered herself middle-class on the basis of her parents, and not believed that her wealth had much to do with what class she fit into. Equating income levels with class levels is more of a US thing, AIUI. (Though wikipedia suggests her family was definitely pretty wealthy: she said her father "was a gentleman of substance, and never did a handsturn in his life", and the family could afford to send her to a boarding school in Paris. So definitely the upper reaches of upper-middle-class.)