In its June 2019 list of new words the Oxford English Dictionary declares “Brideshead” to be an adjective independent of Waugh’s novel. Here’s their entry and usage examples for the new word:
Reminiscent of the style, characters, plot, etc., of Evelyn Waugh’s novel Brideshead Revisited (1945), which depicts the lives of an aristocratic English family in the early 20th century; (more generally) of or relating to the world of the decadent English upper classes of this period.
1961 Financial Times 12 June 18/2 A simple anecdotal narrative, yet it bears the Brideshead stamp clearly enough.
1978 Daily Mail 13 June 19 A mis-spent year at Christ Church, Oxford, spent roistering in ‘Brideshead’ style.
1986 Guardian (Nexis) 8 Aug. The elitism, the class-based superiority, the seductive image of Brideshead decadence beloved of the media.
2018 New European (Nexis) 14 Mar. 21 As a student at Oxford University I had a brief flirtation with the romantic Brideshead myth of ‘Englishness’.
The OED‘s etymology of the new word is also provided: “Brideshead, the name of a fictional castle in Evelyn Waugh’s novel Brideshead Revisited (1945), which was the basis of a popular television adaptation in 1981.”