Sorry for a late response, hope you're well!!
Past papers are an excellent way to revise for your GCSEs, or at least they were an excellent way to revise when I sat them 2 years ago. One thing you must consider, if you want to achieve A* at both GCSE and A Level is:
every student can be resourceful. Every student has access to past papers online, and those that care about their grades will do all of these past papers. However, to achieve the highest grades (especially now and, by judging the trend, it will probably worsen) you can't just be resourceful, you must be smart. Exam boards need to separate those who have merely gone through all the exam papers from those who actually have a deeper understanding and academic capability. The thing is with most past papers, you may notice they ask similar questions year on year and you find yourself beginning to memorise or learn the answers. This then stops becoming a test of your understanding but a test of your memory. This can mean you're successful at GCSE however there are fewer places to hide at A Level!! Something I'm trying to force myself to do (although its a bit late now), and this was advice from a friend who got straight A*'s at Alevel: priorities learning concepts over past papers!!!
I fell into the trap of just learning methods for Maths at GCSE rather than developing my understanding around the concepts: especially with things like graph transformations and vectors. Your teacher will throw A Level work at you and you are expected to have a good grasp on the basics. Same goes with sciences.
My natural inclination, similar to yours as it seems, is English. I took English Literature alongside Maths and Physics. The work load for English literature is sometimes overwhelming however studying something you enjoy is one of the most rewarding feelings. English Literature requires a fair amount of research and debate- this opened my mind up more than anything. I have one final maths exam and then I'm done for summer, then I will be studying Law at (hopefully) University of Liverpool.
Your mindset is the same as mine regarding private schools. Moving sixth forms was the best decision I made, at first I thought it was horrendous as I didn't know anybody but the social skills you are forced to gain make you a much more resilient person. I went to sixth form with the expectation of meeting more like-minded people and having some serious fun that you simply can't have at private schools. The latter was true, and I have met some very lovely people however the judgmental people at private schools- you'll find them everywhere. The same kind of people exist at state schools unfortunately, but they're are the minority
Such a worth-while experience- like a roller coaster, really high highs and sometimes really low lows but you will be so prepared for the transition into University.
You seem to be one step ahead of the game in terms of your education, especially with already researching your degree course! What are your plans? I hope you absolutely smash it, you seem so lovely