Hi nanjan15,
I think the obvious choices might be Philosophy, Politics and Economics! However, I understand many colleges may not offer these subjects, and it's nice to branch out into other areas while you still have the chance.
As McGinger mentioned, certain universities will have specific entry requirements, and they may state subjects that are required to qualify for their PPE degree. Even if you don't know where you want to go, having a quick read around a few university websites might give you an idea as to what subjects are typically expected.
As far as I'm aware, PPE does not typically have certain subject requirements. At Uni of York, BSc (Batchelor's of Science) requires Maths A-Level, but BA (Batchelor's of Arts) does not. This seems to be because the BSc course is much more economics-heavy and will require more statistical knowledge. This is the same for the PPE BSc at London School of Economics and Political Science - so it might be a good idea to think about doing Maths.
Besides Philosophy, Politics and Economics, you might want to think about what a PPE degree covers. For example, Sociology might help equip you to understand societal and structural issues from many different viewpoints. Law might help you understand the role of the judicial system in society and cover some ethical content, as well as teaching you strong essay-writing skills. The same could be said for English Lit which will help you develop strong critical skills, and Maths as I mentioned before will help strengthen your statistical skills for the Economics element.
I studied Sociology and Politics, so if you have any questions about my experiences with these subjects I'd be happy to answer them

Best of luck,
Alex
Student Ambassador