If you're keen on classics I might be inclined to suggest going with A-levels just so you can do as much Latin as possible before your degree. Although IB might be useful preparation in the sense of getting you used to working on a variety of different things from different approaches/methodologies (which is common in classics type degrees as you have language work, literary analysis, historical and/or archaeological options and possibly philosophy), the central feature is the language work and you really want as much preparation as possible for that!
Further Maths is unnecessary for PPE (as opposed to single honours economics) or classics, so I'd probably suggest not bothering with that if you do A-levels. Also note that PPE courses do not require or expect any prior background in any of the subjects (just in maths) and will teach them all from scratch (and the approaches may be somewhat different to the A-level/IB approach anyway so having done those subjects in A-level or IB won't necessarily give you an advantage once you are on the course either) so you aren't missing out on much by not doing those to SL in IB if you did A-levels.
Just for reference as well, as a past IBer ToK was really a bit of a joke - I think we had one lesson a fortnight and I definitely did not learn much of anything about epistemology from it (on the contrary it instilled a sense of solipsism in me for a couple of years more than anything since we never actually learned of any arguments against solipsism...). The entire thing also just hinged on a presentation we did (I think now you guys might need to do an essay for it though) which was also a bit of a joke. You can do an EPQ to do something similar to an extended essay in A-levels too, if you have some idea of what you want to do it on, so you don't miss out on that.