As per above, there’s no real need to do preparatory reading for politics; the course is always taught with an assumption of no prior knowledge (and A level politics does not bear much resemblance to the subject at uni level anyway), and furthermore, it’s quite unlikely (with the possible exception of some classic texts in political theory; see below) that you’ll be assigned any books to read cover-to-cover during your degree - will mostly just be articles and chapters.
That said, reading now certainly won’t do you any harm, and if you are keen here are some suggestions. I’ve divided them into two sections which correspond to the major divide in most politics degree - empirical political science on the one hand, and political theory/philosophy on the other.
For the former, the standard undergrad textbook is Clark, Golder and Golder’s Principles of Comparative Politics. In my opinion, it’s actually one of the best introductory textbooks in any subject I’ve come across, being well-written and striking a good balance between making contact with the literature and not being overly complex and dense. If you just want to see what uni politics is like, skimming through a few chapters that catch your interest may not be a bad idea. Alternatively, if you understandably don’t want to read a textbook over the summer, here are a few suggestions of influential and/or interesting books in the field (note: some can be quite challenging and dense in parts):
Why Nations Fail (Acemogula and Robinson): Examines one of the central questions at the interface of politics and economics, why some countries are rich and others poor. Probably the most readable book in this list, full of engaging case studies and examples.
Patterns of Democracy (Lijphart): Lijphart classifies democracies into two camps, consensus and majoritarian, and argues that consensus is better. Well-written, but has a lot of stats, tables and graphs.
Seeing Like A State (Scott): As its subtitle says, this is about “how certain schemes to improve the human condition have failed” and examines several disastrous attempts at utopian social engineering by authoritarian states, such as Soviet collectivisation and Tanzanian villagisation.
Brexitland (Sobolewska and Ford): A look at the factors, largely sociological, behind the Leave vote and subsequent divides over Brexit.
For political theory/philosophy, Adam Swift’s Political Philosophy and Jonathan Wolff’s An Introduction to Political Philosophy are both fantastic. Swift is more topic-based, while Wolff looks at specific historical thinkers. Will Kymlicka’s Contemporary Political Philosophy is also excellent, though pitched at a slightly higher level than Swift and Wolff. Many people recommend reading classic texts by famous historical political thinkers such as Hobbes, Locke, Rousseau, Mill and Marx as an introduction to political theory, though personally I do not think this would be as enjoyable as the above introductory texts. Most are pretty short, though (personally I think Mill’s On Liberty is probably the funnest read of these).
One book which spans this divide and looks at both political science and political theory is Adrian Leftwich’s What Is Politics? I haven’t read it, though it is often recommended.