Hi Anon,
I am in my third year of studies now and to date I've only had to purchase one textbook and it was for an optional module that wasn't part of my main course for £20, and I later found it was accessible online for free anyway (woops!) It's likely that you don't need to purchase any textbooks, though of course I can't offer insight into their necessity as part of a politics degree. As others have said, even if you do need to use textbooks over the course of your studies your university library will likely have the books you need, and if not most universities offer resources to get the books for free as eBooks or PDFs.
If you do discover that purchasing textbooks is necessary I would urge you to seek out places like eBay or Facebook student groups where you will likely be able to pick them up for much cheaper than you would buying them from retailers.
Hope this helps,
Courtney H