Brilliant. A superb book too - that I read in tempore non suspecto, before the idea of a film was even discussed. I think that Ishiguro's class description is meant to represent a universal model, comparable to the system in Japan (where the author was born).
Added flavor, and even more so since you mention Sunak today, is the fact that both Ismail Noor Muhammad Abdul Rahman Merchant, who was James Ivory's partner and lover, and Ruth Prawer Jabvala are descendants of the British "Raj" in India - where the class system was practiced to its shameful, dehumanizing perfection - by all populations involved, British "gents" and Indian royalties (themselves perhaps somewhat like Stevenses) included.
What masterly richness of ideas, merely in one book/adaptation/film!