As someone that is preparing to take my history, politics and economics a-levels in the coming weeks, and has an offer to read history at oxford this oct, I thought I'd be pretty well placed to offer my view on this.
I was very similar to you when choosing my a levels in choosing between chem and politics, but ultimately I would recommend politics, if you're planning to read PPE.
A science is a huge level of content (that isn't as 'common sense-y' as politics may seem) that although may offer you a broader range of content in your studies, doesn't overlap much conceptually with your other subjects, and PPE, and so therefore isn't as helpful as it may seem imo. I would argue that the three subjects I take offer me plenty of range within a certain area that I am interested in, giving me different perspectives and methods of thinking when looking at a problem, without the need for a science. Sciences are generally useful in conjunction with a humanity like geography, where there is a definite overlap, but otherwise.. make life easy for yourself, pick a subject that isn't as content heavy and as stressful.
I have a couple of friends that interviewed for PPE at oxford, from what they told me they had two interviews, one more focused on politics/philosophy and the other focused on economics, so I would further recommend politics over bio as it will be super useful to have a year and a bit of learning to think in a way that a politics debates are structured, especially if your class does frequent debates as mine did. However interested in politics you are right now, you will undoubtedly be forced to have a wider understanding of not just current events but complex political concepts and ideologies, far from what you would if you took bio (especially if you take four subjects lol, no time). That will always be helpful in boosting your confidence, enthusiasm and range of examples you can reference going into an interview, which is already half of what they want in a prospective student.
Either way, as far as I am aware, a single science at A-level does very little to expand your actual options post sixth-form, as most things requiring or recommending a science will usually want another too.
I apologise for the long, incredibly biased answer, but I hope this helps!