Lady Sidonie's Elizabethan estate might have resembled this one - Longleat House.
Elizabethan Longleat House
" She was determined to come to visit us in Oxford, but girls weren’t allowed in our rooms. So she borrowed her younger brother’s clothes and identity, and came to stay with Quentin for an entire weekend "
These are the Magdalen College residences where she would have stayed with her brother. Edward, Prince of Wales, was at Magdalen at the same time as the characters in the novel, who consequently befriended him.