Hi Mrsuccessful,
I'm not a student in the fields you have mentioned, although I do take sociology modules which have some overlaps with some of the disciplines you have mentioned. On top of the wonderful advice from the commenters above, I'd suggest you have a look at Internet Archive, which is a wonderful source of free books and articles online, and Google Scholar is also a good place to start. These have the additional benefit of being able to reference these sources as they tend to be reputable academic sources, although I'm not sure how much referencing is needed in your fields of study at A-level. Additionally, watching YouTube videos, like the Crash Course channel (which is an overarching channel as it incorporates more sciences), has aided my understanding a lot in grasping theories or thinkers, which I expect is important in philosophy and to some extent in politics as well.
As an even more general advice, I would look into what type of learner you have a tendency to be? Are you learning best with visual, auditory, kinaesthetic or written material, or maybe a mixture of some or all of them? This is important in my opinion in order to use whatever time you have/carve out for studying the most effective, because you are taking in knowledge in a way that makes sense for you. Of course, the more - and more complex - concepts and topics you have to take in, the longer and more effortful it will be, but it's possible to learn more with an ideal method for you in a shorter period of time than struggling with ways that just don't work for you. This also applies to the time if day you are taking on the quest of studying. I know for people with fixed schedules they can't change it is a no choice situation, but if possible, people should be studying at the times when they are most alert, focused and energized. There are tests online for both the learning styles and which time of day you are functioning best, so you can easily find out if you want to/don't know already. For example, if you are a visual type, making mind maps, using colours (which tends to be helpful in any learning) or having pictures/drawing something on the page can help recall immensely.
Please don't hesitate if you have any more questions, and I wish you the best with your academic progression. =)
Viki
3rd year Psychology and Sociology Student