Although it's nothing like actually studying it, when you go to open days try and see if you can go to both the talks about Politics/ SPS and English. It'll give you a much better idea of which you think you would find more interesting, and some open days even do mini-lectures in their talks (Royal Holloway did for English) to give you more of a 'feel' for the subject at university level. Obviously it's not the truest representation, but you might feel more of a natural attraction to one of the subjects. I know when I was considering doing a joint-honours degree with a language, and couldn't decide between German and Spanish, reading the prospectuses and going to the different talks really helped as I just found Spanish language and culture much more interesting. Then I decided I'd prefer to just do straight English, but that's another story completely.
And yeah, like others have said, I couldn't tell you anyone who could just analyse something straight off. I went to an open day at a Cambridge college and they did a mock interview with a third year English student, giving her a poem which had been used on either an exam or as part of the interview previously, and she was struggling to answer the tutor's questions straight off. It's the taking time to unravel a piece of writing and discovering things along the way which is the most interesting part, anyway!