I'm doing OCR for all the sciences and AQA for maths. It's quite easy at first if you're naturally good at the sciences but on thing I regret not doing is going over my notes just to wrap my head around something in case I didn't get it the first time round in class because I never used to do this in GCSE yet it turned out OK but for A-level I didn't realise how much detail you needed to remember for your exams especially in Biology where the content is so much yet. I liked how all the subjects are linked together as it helped to understand topics better e.g. for biological molecules, chemistry helped me understand the inter-molecular forces between bonds that caused the tertiary structure of amino acids in biology when I got confused the first time round when my biology teacher first taught me. Also, even though the specification changed, doing past papers was really helpful (especially the really old ones) as they helped with getting wording right especially in biology which is very specific otherwise you don't get the marks. The same with physics and chemistry as well actually. My school didn't give a lot of homework so I only had to revise (which isn't very long if you just make revision notes or flashcards for an hour or two a day) but they gave us tests every week or two weeks to make sure that we were revising and if we got bad grades we'd have to take extra lessons to help or one-to-ones with a supervisor if we're really struggling but if you keep up with revision there's no need to worry about this. Plus everything you need to know will be in the books the exam boards wrote for the new specification. They'll never ask you anything they haven't told you about so use your resources to the fullest and they usually explain it with examples to help make it easier to understand experiments especially in physics which is very helpful.
For chemistry, I was fine for most of it because it was mostly calculations but when I got to organic molecules I started to struggle and I regret still not going over what I did in class or asking the teacher to repeat it but once I eventually read the book they gave me it explained so easily and Iwas finally able to catch up with the rest of the class.
Maths was still quite easy for me because they get you to practice a lot. I just didn't like the repetition of the things I already knew and for C2 I didn't like doing trigonometry because it got very confusing and was generally my weakest point. I also didn't like graphs of any sort because they took way too much time to draw and the whole thing seemed really tedious to me hence I didn't put the effort come exam time however I took M1 which helped me to understand the mechanics section at the beginning of physics a lot more quickly.
For biology, as I've said earlier, it's very specific and there's a lot of content (we finished the syllabus only two weeks before the exam) so read every section in the book because all the small tedious information they give you is most likely to come up in exams. OCR used to do essay style questions (like 9/10 marks) but now it's shorter and easier to answer as in all you have to do is spew out everything you remember about a mechanism or the steps for something like protein synthesis or just say everything you see on a graph and you can get all the marks but it's also quite hard because they mark in bands now so if you miss out even one thing you could go down a band. You also get a mark for quality of writing (this is for all the sciences btw). They also included maths in biology now and my teachers said it only requires understanding of GCSE maths but when we were given the spec papers for the first time, it was really hard. It's not like the maths was extremely hard but the way that they word the questions makes it almost impossible to understand what they wanted you to do in the first place. I thought it was just me at first but then other people in my class and even other schools complained that it was too hard as well as most of the biology teachers at my school who couldn't get it the first time round and admitted that it takes a lot of time to do which would be impossible to do in the time limit of the exam so I bought a book specifically for the maths in biology that my school provided called Essential Maths Skills for A-Level Biology (it's a CGP book) but the maths in the actual exam wasn't too bad so don't worry too much.
For physics, I found that once you understand the basics of each topic its pretty easy to get an A as they expect you not to get up to 80% so the grade boundaries are normally lower anyway which is to your advantage
but still try and get the higher marks because it makes it easier to answer questions in the real exam under pressure.
Also for all your subjects, I'd print out the specification for each one so that near exam time you can tick off everything you already know and go over anything you're unsure of and you'll be able to see exactly what could come up on the test.
For all the sciences you have to do 12 experiments (only if you're taking A2) so I did six in the first year and am going to do six next year. They were pretty easy and it's pass/fail so as long as there's evidence that you've done it you pass (at least that's what my teachers say) and it doesn't count towards your final grade. However, the experiments can and probably will come up in your exam so you have to remember the steps and for some of them why you have to do a certain step/ what happens if you don't do this step etc. Experiments will also be explained in the new spec books and do the extension questions as they really do help.
Overall, I'd say biology is the most interesting then physics then chemistry to me but even though I like biology the most, it's also the most stress and I'd drop it if I didn't require it for medicine and if I didn't really hate maths. But at the end of the day it was very draining when I had so much to do and not enough time but I still enjoyed it and would do it again since I honestly have no other interest in any other subject as much as I do in science. Also, if you are applying for medicine just know that most people don't get in the first time but I'd apply ten times if I have to get into medicine because I really love the course and love the idea of dedicating the rest my life to helping people in any way I can and if you think you're prepared to cope with the lifetime of stress ahead and with all the issues with the junior doctor contract (which is likely to be worse in the future) then there's not reason not to go for it but anyway, the sciences will open doors to lots of other pathways as well so even if you change your mind on what you want to do, I'm pretty sure you you can go in any field you want with these subjects as long as you can demonstrate your interest to the course you end up choosing with volunteering or wider reading etc. There'll also be lots of opportunities for taster courses throughout the year in yr 12 and you can always get work experience during the half term (as long as it doesn't interfere with studies). Just prepare for failure at first and work your way up because not everyone can keep up with all the sciences at first but you get used to it and get into the habit of studying everyday from the beginning and you'll be fine (making a timetable adjusted to your school timetable helps as well) but even if you can't study everyday, don't worry because it's never too late to start studying properly and no knowledge is lost (as my dad would say) and don't compare yourself with other people around you because the way you learn is different and usually unique to everyone else so go at your own pace and as long as you're making some kind of progress there's nothing to worry about.
Good luck in yr 12 and figuring out what you want to do.