The scene of the tents in the quad of Ryder's college (in preparation for a ball) was filmed at Hertford College, Oxford. That's the same college that Evelyn Waugh attended in real life. Students there can point out a ground-floor window in the Old Quad where 'Sebastian' leaned in and threw up, which is how Ryder and Sebastian meet in the book.
The character of "Anthony Blanche" is closely based on a real-life student named Brian Howard, famous in Oxford for having declaimed T.S. Eliot's "The Waste Land" through a megaphone from a college window (something his fictional counterpart also does). Howard was a budding poet whose writing was, in student days, regarded by many as extremely promising. He had many friends who became famous writers, and figures in many literary biographies; but his literary output after Oxford consisted of only one small volume of verse. Despite his surname, Brian Howard was not a member of the Howard family who owns Castle Howard, where the filming of Brideshead Castle took place.
The jeeps shown are left-hand drive, whereas British vehicles are right-hand drive. That is historically accurate. The USA provided jeeps to almost all of the Allies in World War II, including nearly 105,000 to the British Empire.