“SO, do you have any advice for a JC1 kid? :p Like a “dear 18 year old me” kinda thing haha”

Sure, you asked.

1. Meatbag management is priority number one. To do well in school you need your brain to function well, and for your brain to function well you need your body to function well. There are a few critical components that go into this: Sleep. Nutrition. Exercise. These are make-or-break and non-negotiable. If you’re serious about doing well, make these your priorities. Make sure you eat a hearty breakfast every morning. Do not fall into the trap of being sleep-deprived at school. Exercise regularly. If you’re a skinny dude, hit the gym and learn how to do squats and deadlifts. You’ll thank me later.
2. Do your goddamn homework. It’s painful and stupid, yeah, but it’s less painful than if you don’t do it. Turn it into a game. Schedule time for it and respect your schedule– you’re training to become a high-functioning adult who can Get Shit Done. And reward yourself when you’re done. Enjoy that guilt-free stuff.
3. Break things down into sub-components. If you schedule time for “study”, you’re not going to do anything. It’s too vague. You need to be very precise. Pick a specific subject in advance, and pick a specific topic in advance, and decide exactly what you’re going to do.
4. Work backwards from the end goal. One thing me and a lot of my friends did stupidly was think of “studying” as “sitting around with books”. That’s like training for a marathon by putting on your shoes and just walking around as you feel like it. That doesn’t work. Exams are an ordeal, and you need to train for them. For your A Levels, you’re going to have to sit on your ass and focus for 3 hours at a stretch. That’s something you need to slowly work your way up to. Do practice papers and zero in on your weak spots.
5. Choose your friends wisely. It’s going to be tempting to just hang out with whoever is familiar or cool or likeable. The smart thing to do is to seek out people who are driven and work hard. The earlier in life you do this sort of thing, the more you enjoy the compounding benefits over the years.
6. Have a good social life. I’d recommend reading things like How To Win Friends and Influence People and The 48 Laws Of Power. Young people are usually socially inept unless they’ve got really enlightened parents and families to help them out. Even then, every adult looks back on their teenage years with some cringe. If people tell you secrets, KEEP THEM. If you make promises, keep them. Don’t say mean or nasty things about anybody. Be kind, friendly, helpful. If you find yourself about to have sex, please use a condom. The peace of mind is worth it.
7. Pursue your personal interests with intensity. Whatever it is you like to do. Guitar? Basketball? Draw manga? Write fan-fiction? Whatever it is that you like, DO IT. Do it hard and do it good. Manage it like a project, set targets and goals, be prolific. You won’t regret this. Some people in their 20s have awesome skills. Some simply don’t. And it boils down to how much focused effort they put into whatever they were doing.

Good luck! And don’t take it all too seriously. It’s important, but it’s not THAT important. Be nice to your parents if you can manage that. Become a person people respect and admire and rely on. It’s worth it.