The excellent talks by Oxbridge dons ( maybe you, LRD) given to DS's school pointed out in no uncertain terms that they didn't give a flying f about whether you had grade 8 flute - their example. They were interested in whether you were good at and enthusiastic about their subject.
They pointed out some important things, though, like:
Read the content of the course. If it says it's mathematically based, it will be. If you don't want a mathematically based course in , oh I dunno, Ancient Greek, don't apply here, apply somewhere else. ( this is important. One of the best students at DS's school went to Oxbridge and discovered that the course didn't suit him, too x where he wanted y. He has since left.)
It is very competitive. If you don't get in, that's not a comment about your moral worth or anything else. And lots of dons didn't do their first degrees there.
The Cambridge guidance is excellent, tells you what is mainly expected for each course re A levels and any other qualms that might help, eg for the maths / economics courses. Didn't look at Oxford this time round.
Decide whether you want to be on a campus university or not.
Decide what you find helpful in teaching practices. Some people find the tutorial/supervision system too frightening.
Decide whether you can cope with interviews for admissions. Some can't.
Decide whether you can cope with the pressure. The terms are slightly shorter than elsewhere, a lot of the results are based on final exams, that may not suit you.
Don't be put off by the nonsense photos in the press. Yes, balls happen. And? They happen elsewhere too. I'm quite sure there are the equivalents to the Pitt Club and the Bullingdon elsewhere as well. And lots of students like trying out different personas when up. So they may be dead posh for a period and dead unposh for another, and why not, it's a place to explore ideas and who you are. And the range of students is great because both take huge care in their application systems.
On the which college question, often it used to be used as an icebreaker. But students then saw it as a determining question which it wasn't.
LRD, simply delighted there is now some centralised training, asked the question ( quite often) when working for my previous employer ( had a good reason for doing so) and was treated with contumely. Even if it's not always followed. Standardised list of questions?