I'm studying Economics at Cambridge, and my boyfriend is a PPEist at Oxford, so I have a pretty good idea of the differences in the course.
Because Oxford's course is split with Politics and Philosophy, you're unlikely to gain as deep an understanding about Economics as at cambridge since you'll cover less ground in the subject. (The teaching style appears also rather different from that at Cambridge.) However, PPE is designed as such because the three subjects go well together, and you can specialise as you go on. My boyfriend is essentially doing Economics now with less courses in philosophy. Theres also less maths involved in their courses (and econometrics isn't compulsory). The downside of this, is that his tutor wouldn't support his application to do a masters in the US because he thinks the courses there would be too much of a jump from the work he's done in Oxford. (And also because his tutor is a git.)
I take it from your messages you're more interested in Cambridge(?)
If you're worried about Maths, make sure you choose a college that doesn't require STEP papers in general since they tend not to care as much about it. Maths *is* important for economics and I do advise taking Maths A2. You can avoid doing maths in your course to a high level, but it is VERY useful for understanding a lot of the course content (particularly microecs).
It's unwise to think that having business and economics a levels make a stronger application than one with a lot of maths, though it does show a commitment to your subject. Alevels which are 'watered down' versions of degrees are, IMHO, pretty useless. A friend of mine had an offer for Engineering, but the 3 A's were required in her 3 of 4 subjects which WEREN'T engineering! She would have gotten in even if she had failed it. I didn't do economics either, but the entire course was probably covered here within the first few weeks!
Finally, you want to know how to increase your chances. I might get flamed for saying this, but I think girls' colleges are easier to get into. However, whichever college you choose, do make sure you can give a good reason for applying there! New Hall has the lowest # of direct applicants per place and the Econs fellows make a REALLY big effort to make your interview experience as un-painful as possible. This gives you the best chance at performing well, and this is the main reason why I think female colleges are easier.
This was the advice my school gave me. In hindsight, I now wish I had applied to a mixed college because I think I would have been good enough at interview elsewhere and could have gotten in through the pool otherwise. If getting into Oxbridge is more important than the college, then do do everything you can to improve your chances. Most importantly, *talk* to a lot of people and develop your interpersonal skills. This is the thing that will most help you at interview!