Hi, I am just going into year 13 (at a comprehensive school) and am also looking into applying for HSPS. At GCSE I got 2 A*'s, 6 A's and 3 B's. This year I have AABB (though I am getting one of the papers with a B re-marked and suspect that it will go up; as it is my best subject). My average for AS is 89% (the Cambridge average is 90% [which means there are people who apply with both higher and lower than that]). I have 100% for one of my subjects and 90% for another. Generally, if you have an average of 85% or above you should get an interview. So, regarding GCSE's I wouldn't worry about it; I did well being in a similar position to you with GCSE's. If you make it known to your teachers that Cambridge is your goal, they should really support you (essentially all my teachers at did). Things like the school reference are very important, so ensure that you try your best to impress teachers with extra reading and work throughout the year and the reference should be good. Also, Cambridge tend not to care about extra curriculum activities and volunteering etc; as it does not measure your academic ability. Fundamentally, if you get an interview, it is you as a person they want to know, and your chances of getting in depend on whether
they want to teach
you.
I'm not sure about interview, but if I get one I can let you know how it went! Also, not all colleges sit tests. Here is a link to the colleges that sit tests, just click onto 'entry requirements' and then 'admissions tests and written work':
http://www.study.cam.ac.uk/undergraduate/courses/hsps/ Also, I would probably add another 'facilitating' subject in the place of any but English Lit. Facilitating include: English Lit, Maths, Chemistry, Biology, Physics, Languages, History and Geography. If your strengths lay in essay writing, I'd do history. If you are comforted by revision, have a good(ish) writing style, revise enough and find it relatively easy to understand key concepts and have the ability to link things well, then Geography is a good option. I did not really enjoy physical geography much but loved human geography and was able to get 100% in both exams. So, for me it was a very good option and is also relevant to HSPS.
I hope this helps!! And, just out of interest, how did you do in your GCSE's?