I personally prepared for o level physics and maths by binging a college algebra textbook, watching mit ocw stuff and reading what the physics olympiad people recommend (in particular the advice by Kevin Zhou on his website. Sad that there isn't a big physics olympiad community that is UK-based, but the US-based one has developed really fast). It covers most of what I need, and I just need to plug in my gaps in understanding when I do my maths and physics homework.
So if you wanna prepare for gcse, I recommend you just learn and understand cool stuff (anything for that matter that interests you) at a deep, intuitive and familiar level right now, so that during GCSE classes you can use your understanding of certain topics to help understand better the whole subject, and the months leading up to the exam, you can easily recall and use the knowledge needed. Remember, you just need to get a good score on the day of your gcse, and you won't remember the quadratic formula or how to invert matrices a few years later if you learn them now. What is really important is that you have a good understanding and intuition of interesting and relevant topics that you like.
Also, learn some good studying habits too. You can practice writing notes for easy reference later, sleep and rest well, you know, the usual.
I'd say go learn all sorts of cool stuff you find really interesting and explore. You can learn to code and write computer programs to do linear algebra homework like Evan Chen hahaha, or prove calculus rules using limits, and watch how other people do cool things; to do the last step you'll need a good network of people that is really into learning more stuff, and I'm lucky to meet loads of these people online at AoPS and Physics Olympiad Discord Server (AoPS is dead though, ISODN is now the new active place); they are the same age as me and we like to chill. Never had I ever thought of preparing for A Level Physics because I know that (having seen the syllabus) I am familiar with 2/3 of them and I just have to practice past papers and remember some good heuristics the few months before the exam (but by the time I'm doing A levels I wanna complete core undergraduate physics: Taylor, Griffiths EM, Double Blundell, and Griffiths QM and learn some chem from Atkins. I probably just need to revise a bit, practice a bit and plug anything missing a few months before the exam and I'll be ready.
IMO someone should popularise the US-based olympiad community to UK students. Good peers are always important, and they really help me a lot as supportive people and also sources of advice that I couldn't possibly get from my teachers here.