I would really recomend you read some separate introductions to philosophy, politics and economics. Although the guide to ppe provided by oxford says to read an introduction, newspapers and the economist, the 7 people who got in at my college all seem to have read a book from every philosopher under the sun...obviously a small exageration, but i would recomend reading
Hume, An enquirery concerning human understanding
Developments in British politics (latest edition)
Philosophy A very short introducion
Think Simon Blackburn
The Economist weekly
Some sort of economics introduction to get some understanding of the basic concepts...for example in the test game theory did come up, and i hadn't really come across it and it was a nightmare! But if you're taking it at A level then you're probably already in a far better position than i was
Another book often recomended is Russel's Problems of philosophy but i personally didn't read that as i thought if everyone had my answers might be a bit generic (a stupid excuse for laziness probably)
Also consider reading some marx - communist manifesto and rousseau - the social contract, for you political philosophy
Hope thats been helpful
Will