As for the Brideshead image, it's a historical hangover which most people have sensibly slept off. The prevalent mood of the college community, and certainly of the classicists, is 'work hard, then play hard'. The different courses offered, and the qualifications needed to tackle each of them, are described in the 'Classics at Oxford' booklet. You will have to sit either one or two written tests, each lasting not more than an hour, when you come up for interview, and you will have been asked to send in one or two pieces of school written work beforehand. There will be two interviews in Christ Church, mainly concerned with aspects of your academic work and experience with Classics.
Interviews normally last half an hour.
The Classics tutors will write to you after New Year giving some advice on work to be done in the summer vacation, in most cases after you have done your A-levels. Therefore it is important not to commit yourself to travel or paid employment which occupies the whole of that vacation. The Tutors do not normally make difficulties about this, provided you have a reasonable project in mind for the period in question. It is always a good idea to discuss this at the interview stage. We can only consider deferring a place before offers are made.