I did Edexcel Politics and ended up getting A*. My college gave us textbooks we needed to have but honestly they weren't that useful. Textbooks will help you with learning theories and political science, the sort of bits you need to understand and not just know. But real life examples are just as, if not more, important because that what lets you back up all your arguments in your essays (you NEED to do this to get a decent grade) and is also useful outside of exams in debates and classroom discussion. So it's really really important to follow the news, watch it, read it, discuss it. Mainly discuss it. In terms of revision, making essay plans for potential essay questions is very useful. Come up with a few points for both sides of an argument and find examples for each point. I also find revising and making essay plans with a friend helpful as you get a different perspective.
Ultimately, in my opinion Politics isn't just a subject you grind, study all the facts, and come out with perfect grades. Maybe it is for some people but I found the people who did best in my classes were the ones who were really interested and engaged in politics outside the classroom too. Read about politics and discuss it with people, listen to other opinions because even if you think they're ridiculous they'll teach you a few arguments you can use in your essays. If your sixth form / college has a debate society or a politics society consider joining that.