hi, i'm in year 13 and take maths, further maths, physics and chem 🙂 i think it's definitely manageable to do four a levels as long as you manage your time well, and considering you'll be sitting maths in year 12, i think it would be better for you to sit 3 a levels in year 13
to be honest, both physics and chemistry are difficult but in different ways, and there are advantages to both (i can talk about bio a little from what my friends have told me). i don't think that there would be much of a university preference between the sciences for the course that you're planning to study since you're not aiming for a super science-y course - so choose the one you think you will enjoy the most and do best in
physics has the greatest overlap with your other subjects so reduces your content load in that way. if you take further mechanics as one or both of your further maths options, this is even more relevant and you will find that you cover the same topics in normal, further maths, and physics, which means you end up with really solid practice in these topics and have less content to learn. physics is hugely different from gcse imo - i actually found it quite boring at gcse but the topics you learn become so much more interesting at a level. it's really cool how in exams you actually apply your knowledge to very real scenarios, so it feels like you're doing 'proper' science. it's a great subject if you have a curiosity for how things around you work as they do, and becomes really great as you move into year 13 and start to see all of your knowledge coming together. that being said, it had the largest step up from gcse of any of my subjects (i didn't even take further maths gcse, and i found further maths as to be less of a step up from normal gcse maths than physics was) so you do have to be very committed to do well in it. it's notorious for being hard for a reason, but completely worth it if you're passionate about it
chemistry has a little overlap with maths but nothing at all like physics. the first half of as chemistry is mostly taking your gcse knowledge and expanding on it - that being said, a lot of people did struggle. the second ~ half of year 12 is organic, which is probably the biggest difference from gcse as you just do so much organic. i found it really fun and my favourite part of the course in year 12. out of my subjects, there was the largest jump from year 12 to 13 in chemistry imo, because suddenly you're doing completely new topics, and it became really, really maths-y. but, considering you'll be doing double maths, this should be alright for you. the biggest problem i can think of for chem is the grade boundaries. for context, to get an A* in physics you need ~75% (aqa) but for chem, you need ~90% (ocr a). that being said, this does reflect that chemistry papers do tend to be quite repetitive whereas physics papers require more problem-solving. i wouldn't say it's been that heavy on memorisation - more content than physics, but completely manageable
i can tell you a little about biology based on anecdote from friends. it's significantly harder than anyone in my year expected, so don't take it because you think it's the 'easy' science. the grade boundaries are similar to physics (ocr a) and this is reflected in that the papers are more application-based than chemistry are. there is a huge amount of content, and some of it is hard, some less so. there's a significant proportion of chemistry, especially when you learn about biological molecules so it's quite different from gcse in that respect.
sorry for the really long reply but i hope some of it is useful
