Can you narrow it down at all? Is she a good all-rounder who cannot choose between physics, modern languages or Eng Lit? Or is she actually at a point where despite her academic ability she is ready for a break in formal education?
I was ready to leave school at 18 despite being very academically able. Went back at 22, completed three degrees in quick succession and am now an academic. I worked for the intervening years in a related field which I loved, but at 18 I didn't need or want the uni experience. I wanted the independence of supporting myself financially and doing a job I cared passionately about.
But you say your DD does want to go to uni. In which case she needs to think about why. Because her friends are going. Because it is fun. I think those are good reasons, but they may make study pretty dull. Going because it is what people do at 18 is a lousy reason.
And going because she will enjoy further study and is just overwhelmed by choice is a great reason.
Not knowing what she feels passionate about yet is really normal at 16/17 and should not be a cause for alarm. The summer open days next year should help. It is OK to consider a range of options and when I help out at summer Open Days we quite often chat to students about the strengths of doing my subject versus a range of other subjects. I don't see my job as to sell my subject over the other options, but to explain what studying it is like at degree level so they can make an informed choice.
In the meantime it is great that she is doing the IB and keeping her options open.
Finally when visiting universities and looking at degree programmes ask about modularisation and how easy it is to take courses in other departments or consider joint honours. Oxford and Cambridge might both be good for her, since they allow students to study widely across disciplines, but so do many other universities. I don't want to name my place but we're relaxed about this, and so are most unis which have switched to the modular system, provided 2/3 of the courses are taken in the graduating discipline. So she wouldn't have to abandon her passion for philosophy if she decides to study maths.