I would think of a particular historical period or figure you are interested in and research it fully ( books, podcasts, documentaries, museums ( also speaking to the custodians who sit in the rooms and are very knowledgeable). Some may even have archives you can visit, but you will need special permission.
As for Spanish, it is important to familiarise yourself with the mlat. Each section will have sentences revolving around a certain grammar point, so please get a grammar book with sentences to translate, and hopefully ask your teacher to mark them. As with history, read, read read Spanish newspapers and literature so you will come across the kind of words you may get in the mlat. Most languages a levels focus on holiday/ environmental vocab, so when the word “saucepan” came up on the German test, many were stumped! I do recommend buying usborne 100 first words in Spanish, which has an illustrated double page with kitchen vocab in it, plus any other basic vocab.
I would recommend starting a vocab file and when you come across a word or phrase you don’t know, write it down. When I was studying french, if I found a really cool phrase I would write it down and memorise it so I sounded more authentically french. I can still remember some of the beautiful phrases Princess Stephanie of Monaco used in her 1983 Paris match interview, for example! Once you have this file, spend 10 minutes per day learning the vocabulary. You still need to do 80% schoolwork and 20% Oxford.
I would look at Spanish poetry, as you may be given a Spanish poem and be asked to analyse it. If you want to, you can start by getting a book with the Spanish on one side and the English translation on the other. Maybe again, your Spanish teacher could sit you down and you can analyse these poems together. I would rather you just read two or three books and really analyse them well and appreciate them rather than 20 books and just whiz through them without reflecting on style, characterisation, structure etc. Just really enjoy the book.
As for college, my son’s college, Exeter, has links with the Spanish royal family ( my son took a photo of ex queen Sofia walking around main quad). They even have Katharine of Aragon’s prayer book in the library. Plus it is the best looking college in Oxford ( check out the amazing view from the mound, and the “hot” chapel)
Best of luck and let me know how you get on.
As for what to read in Spanish, I am not very familiar with the literature but Google 100 books in Spanish you must read, then look at the mini blurb and make your choice. A lot of the tutors favour more modern obscure literature. Just read whatever you fancy, as it is supposed to be your individual journey and almost enjoyable academic development for its own sake.