To study at Oxbridge/Russel GroupWatch
Government and politics isn't a facilitating subject. But a facilitating subject doesn't mean a 'hard' subject, it means that it forms the basis of other subjects. So for example History is a facilitating subject of law and politics. Do you see? So don't worry about not doing facilitating subjects, as politics and philosophy are two highly regarded subjects that are not facilitating.
I want to study History and Politics, I chose History, English Literature, Philosophy and Politics. These are perfect subjects for history and politics or law, because they are humanities and basically the foundations of law. So, yes while you may not be taking 'Chemistry, Maths, French and English Lit', the subjects you choose should be ones you enjoy and related to your chosen university course. Cambridge states that Government and Politics is a valuable a level for someone hoping to study the humanities. Plus, if you're wanting to study politics, having that basis of knowledge will be really helpful and will aid you with your PS. Unlike subjects like law, politics does prepare you for politics at University. It teaches you about the British Constitution etc.
The A-levels you chose are good as well, though.