To be honest, it's most likely the case that politically-minded people want to do PPE, rather than that the PPE is particularly good at preparing people for political office. Certainly, most politicians didn't study PPE at Oxford. If you don't like maths (which is odd, given that you say you're good at science) you'd probably be better off looking at the history and politics course. As the Oxford website says, history and maths are both useful for PPE and if your maths is weak you {probably/very probably/almost definitely} won't get in.
It doesn't matter to Oxford whether you do your A levels at a sixth form college or a school sixth form. Just try to get into the best place you can near you.