Well good for her. She'll probably change her mind 1000 times between now and then.
Personally, I'd not focus on it much right now, in case she goes off the idea. Instead of homing on Cambridge, help her develop the key skills and knowledge to get her in there. My DH read English at Cambridge - but that was last century, and I was at Oxford, so our info is outdated, but the key ingredients for us were:
read incredibly widely - way beyond the curriculum and related texts.
Show interest in literature that isn't covered at A level but would be at uni (e.g. if she read's Armitage's Gawain translation and Heaney's Beowulf and some of the original texts too.)
Go to the theatre, as much as possible, especially to see classics and Shakespeare.
Make sure she knows her classical mythology and Bible stories. Kids versions of these are fine for now.
Most important - encourage independent thinking and independent critical reasoning about a text. That means she doesn't trot out what Brodie's notes says about Romeo and Juliet or The wasteland, but has her own original views and can back them up with examples and scholarly, logical reasoning.
Modern languages help enormously - as would Latin or Greek.
Stick to pure subjects at A level: English, French, History, Maths, Music, maybe Drama rather than Media Studies or similar modern blended subjects that are seen as soft options.
Excellent GCSEs needed in all core subjects - including Maths.
You could take her to visit too, - go for a punt and wander round the colleges. And take her to Oxford too, just in case...
HTH.