There's no formal requirement to read anything before you arrive. Your module leaders will explain everything you need. But if you want a recommendation for interesting material over the summer:
Francis Fukuyama, The End of History and the Last Man (1992) is a story of the end of the Cold War and what it meant for global politics. It has a lot to disagree with (particularly given the thirty years of history since), but has a clear overarching argument, and ranges from philosophy to economics to politics in making its case that western style liberal democracy is the best system available.
Steve Richards, The Prime Ministers (2019) is a chatty account of the Prime Ministers in Steve Richards' living memory (so, since the 1960s). A political journalist giving a nice recent history of modern British politics.
Rachel Reeves, Women of Westminster (2019), is a current MP reflecting on the women that sat in parliament before her, how they made their careers, and the challenges they faced along the way.
All three are available in audiobook form, if preferred.