@jneill thanks for tagging me in
I'm a current A-level philosophy student who has applied to Cambridge. I will be receiving my rejection letter this Tuesday, but that's more to do with my performance at interview than my A-level subject choices!
At AS I took: geography, history, English literature and philosophy. I then dropped geography after AS and carried the other three on to A2.
I don't think there are any specific A-levels which would be vastly more helpful than others. The candidates I met who had also made it to the interview stage were taking a wide range of subjects. There were humanities, sciences, maths etc. Oh, and I was the only one studying philosophy at A-level so not taking that won't disadvantage you either. Either way, they will be most interested in seeing that you have done wider reading in your own time.
Having said that, if your school does offer a philosophy or philosophy and ethics course, that would be a good A-level to take, just to make sure that it's what you want to study for three years! If not, I know several philosophy applicants had religious studies and there is some overlap between that and philosophy of religion: ontological argument, cosmological argument, problem of evil etc.
Essay subjects such as English and history are useful because it's an essay-based course. However, (and this is something which I did not know before applying - I am TERRIBLE at maths) Cambridge's philosophy course is very focused on logic and it is a compulsory module in the first two years. There is also a logic component to the admissions test. Formal logic in particular is quite mathematical so having maths or even further maths will help you to think in the right way. But, this is clearly not a requirement either.
There are no subjects which Cambridge explicitly asks for, so on the whole I would recommend that you take what you are best at because at the end of the day if you do get an offer it's likely to be something like AAA*.
What I would say is, make sure that the subjects which you choose are on Cambridge's list of facilitating subjects, found here:
http://www.trin.cam.ac.uk/admissions/ug/apply/a-level-combinations