Yellow, yes, you are right to give more details. What I meant was that the days of one tutor, one decision are over in everything, though I understand English is still much more collegey than most.
wordfactory - she should go to Open Days - colleges have them and so do faculties - and go to the stuff laid on. Typically there are lectures in the Faculty and a chance to meet college tutors and ask questions. All of it is well worth it.
Colleges and subjects - a lot of misunderstandings here. it's true that some places kinda specialise - Trinity Cambridge and classics, for instance - but in general the best plan to is to research the teaching staff at a college. While maths and physics are taught pretty centrally, most humanities subjects are still mostly college-based, so the people on the coll website ARE the people with whom she will spend most of her time. (History and classics are now much more option-based, that said.) Some reputations are a bit old and whiskery - Wadham for example used to be regarded as the ne plus ultra for English because Terry Eagleton was there, but he hasn't been near the place for 15 years. Best as said to look at the college websites, do a bit of googling. She could research one college a week from now till next October, perhaps? thestudentroom can be helpful - with a pinch of salt...
A key factor is cost - colleges vary a LOT in their charges for accommodation and how much they have on hand.
Very generally, the hardest places to get into are apt to be the smaller stone colleges. People are put off big rich colleges like Johns and Christ Church by the image problems, but think say Hertford or Exeter looks friendly. Anywhere more recent than 1750 is often an easier bet for entrance. A few places still have regional links - Exeter and the West Country, Keble and the North, Jesus and Wales - and the last name is so offputting that it's an easier bet than some - but that doesn't mean someone from London would be rejected by any of these. Just there's sometimes a few tiny pots of dosh knocking about :). Think Jesus has monies for the scions of Welsh clergy...
Message me privately if I can be of more help. She should absolutely go for it if she's prepared to work hard and has 3 As predicted at A2.