It was written during the war which is probably one reason why it's so full of food, and most interesting from my point of view, drink. There's a dinner about half way through where Charles Ryder is dining with Rex Mottram, who we are given to understand is not quite a gentleman which has particularly stuck with me. Rex is paying, but Charles has chosen the dinner. It finishes with a fine old pale cognac served in a tulip glass, it's stated as being a year or two older than Rex - so at a guess I'm thinking 30 years or so. Rex declaring he knows about brandy demands a balloon glass warmed on a spirit burner, disparages the original cognac, and ends up with something dark and syrupy.
Whatever the reader knows about the respective merits of different cognacs the message is clear - Rex is an outsider who lacks class, Charles, the restaurant staff, and the rest of us are now judging him for not being able to recognise quality. I think it stuck because it feels like the distilled essence of Waugh to me, containing everything that I find both attractive and repellent about him.
Brandy is the spirit you get when you distill wine and it can be made anywhere. Cognac and Armagnac are the most famous brandy producing regions in France, both are brandys but there are some key differences between them. Armagnac, which I believe is the first recorded distilled drink in France, is made in a sort of continuous still, Cognac is pot distilled. The way the two are regulated is slightly different too, and whilst Armagnac comes in vintages, generally speaking, cognac does not.
The initial V.S, V.S.O.P, and X.O that appear on bottles stand for very special, very superior old pale, and extra old. With cheaper brandy the dark colour is more likely to come from caramel than from oak barrels. I'm not quite sure if Charles is drinking a particularly old V.S.O.P or what would have been the equivalent of an X.O, but either way we can be sure it was expensive.
Again, very generally speaking, pot distilling gives more character to a spirit than continuous distilling, it's also a more expensive process which is one reason why the difference between an X.O Armagnac and an X.O cognac is upwards of £100 a bottle (though there are cheaper supermarket own label versions around). It's also worth noting that not everyone likes 'character' and that many prefer (for want of a better word) the smoothness of Armagnac against the 'bite' of cognac.
With budgetary considerations to the fore, my Brideshead recommendation would be an X.O. Armagnac (Waugh might not entirely have approved - though then again, he might - if it seemed exclusive enough), sipped, of course, from a tulip shaped glass.