Preparing for uni course

Firstly, try not to be anxious. Most people will be starting the course with minimal relevant knowledge. Have you tried emailing your department to ask if there's a reading list or any recommended preliminary reading? Most universities will provide you with a list if you ask. I would also recommend looking for relevant Very Short Introduction books (A Very Short Introduction: International Relations would be a good start if you haven't already read it). Those books are often on the reading lists of universities because they're aimed at a university level and so can be more useful than A level texts. But don't worry if there are things you can't understand or if the writing seems dry (some books in the collection are better than others).