I discovered my interest in the language from chatting with the French assistants at my school and then going to France to work on a farm in the summer after GCSEs (and every summer thereafter). I then started to look at the culture of France when I decided in year 11/12 that I wanted to do French at university - I love English lit anyway so reading in French was a natural extension.
Don't worry about applying for joint honours - if you do well in one subject but not in the other, they frequently give offers for single honours (So if you're good enough for History but not for French, that won't necessarily stop you getting into Oxford).
Well the MLAT
is around A2 level, but they don't expect people to get full marks or anything. The average mark for offer holders is around 70% for French, according to my tutor. They test the same things every year: irregular verbs, uses of the subjunctive, some vocab, use of tenses, use of prepositions. When I took the test I would say I had a level beyond A2, but only because I panicked and basically made notes on every chapter of Ferrar's 'A French Reference Grammar': this is totally not necessary (though helpful) and there's actually a great variation in grammar ability in 1st year. Some people actually did pretty awfully at the test and still got in because of other factors (like their interview).
The course is very literature based, but as a dual honours student you could actually reduce your finals papers to one literature module, if you opt for linguistics/advanced translation papers instead. In first year you will have 2 literature papers covering around 10 novels/plays/poetry collections etc.
I would recommend trying a variety of genres and works of different periods:
Theatre:Phèdre by Racine is a classic example of 17th century tragedy.
Le bourgeois gentilhomme by Molière is a good starting point for 17th century comedy.
More modern stuff: Ionesco, Sartre, Beckett - their plays are simple to read but hard to understand on a thematic level.
Poetry:Les fleurs du mal by Baudelaire. You can read it here, with English translations:
http://fleursdumal.org/1868-table-of-contentsIt might be worth buying a poetry collection like this one:
http://www.amazon.com/Penguin-Book-French-Poetry-Translations/dp/0140423850/ref=sr_1_1?s=books&ie=UTF8&qid=1402048366&sr=1-1&keywords=penguin+book+of+french+poetryTry to think about how form informs thematic considerations in the poems. You don't need to know technical vocabulary, but doing a bit of research on French versification might help with this goal.
Novels/Stories:Candide by Voltaire is quite accessible.
I loved
Le Grand Meaulnes. It might be a bit of a challenge, but it's a nice story.
Un sac de billes is also good. (It's about two jewish kids trying to reach the zone libre in Occupied France).
The joint honours people don't seem to have that much more work than other courses (at least for Modlangs + English) but the work can be a bit more unevenly spread due to the organisation of each subject (so one term my friend doing French and English only had 4 essays to do in one term and then the next he had like 14).
As for improving your language skills: try to speak in French as much as possible. A summer course is a good idea, though it would also be good to speak French to natives outside of the classroom if possible, because it really boosts confidence.
The accuracy think is just a matter of practice - be methodical in checking your work and make sure you're grammar is solid - after a while you will just naturally avoid/spot errors much quicker as you get a greater 'sense' for the language. As for vocabulary, I've become addicted to memrise.com, which turns vocab learning into a sort of game with leaderboards etc.