The plot of Brideshead Revisited is mainly centred around the period from 1923-late 1920s, with the final episode taking place in 1943 or 1944. The novel is a classic which is well worth reading, but if you are particularly short on time Wikipedia does a nice summary of the plot and themes. and there is always the classic BBC adaption and the recent film version to watch for inspiration.
Any attire from between 1923-1944 would be perfectly appropriate, though the tweedy, conservative, upper class between the wars look fits the theme much better than the flashier 'Roaring 20s' look.
There is lots of scope for imagination within the upper class look, whether you are a historically accurate fanatic or just want to go for 'effect'.
For the ladies: think dropped waists, below the knee wool skirts, thick wool tights (it is winter after all!) and the indispensible cloches, though knitted caps and broad brimmed hats were also fashionable at periods in the 1920s and 1930s.
For the gentleman, the film adaptions feature lots of three piece woolen suits and knitted cardigans. You could also go with the final chapters of the story and wear a WWII Army uniform (and who knows, there may have been a few Kiwi soldiers stationed at Brideshead!).
In additon the Art Deco Society does a great rundown on how to be a Gatsby man
And finally, for both ladies and gentleman Te Papa's collections online has hundred's of Leslie Adkins photographs of the 1920s, with fantastic casual images of his family.