And on that note, I should probably talk about my experience with the subjects I take.
Biology - we start off with biological molecules and cells (first two topics are usually taught simultaneously). It is A LOT more detailed than anything you've ever seen at GCSE and there's quite a bit to remember, which is an understatement. However, it is also very interesting. You'll see how everything actually works inside organisms and cells, because a lot of the time at GCSE you just assume and accept the content you learn, but at a level you begin to question everything and there's a bit of philosophy on evolution and quite a bit of maths and stats which I didn't expect to like and a lot of biochemistry - a lot of crossover with other subjects. Conceptually, the content isn't too difficult to wrap your head around, but there's a lot of it. Also, when you complete exam questions, you will see how specific and picky the mark scheme is, you miss out a key word you lose the mark, but you'll soon learn exam tricks and how to apply your knowledge. A LOT OF PRACTICAL WORK, AND I MEAN A LOT. I've probably done more practical in biology and chemistry this term than 5 years of secondary school! Also, a lot of opportunity to go beyond the curriculum and do independent research.
Chemistry - A LOT OF MATHS, at least for the topics I'm studying right now. Great if you love maths like me, not if you don't. There's quite a lot of crossover at GCSE, which is good as it just expands on your knowledge you already have. Some of the content is quite conceptually difficult and takes some time to wrap your head around, but personally, as of now, chemistry is slightly easier than biology since for me there's not as many key words and specifics we need to memorise. For some reason, I love the biochem but I'm not a fan of the organic??? Hopefully organic gets better soon.
Maths - again, a lot of crossover with GCSE, for the first 8 weeks of sixth form the content was just GCSE higher content. Calculus is where all the fun begins! If you've never done calculus, you're missing out. Quite a heavy workload but definitely manageable if you love maths.
FM - we started with decision maths, which isn't that popular of a module (I think more ppl do stats and mechanics, we start stats later in y12). A lot of content is very different to what you have learnt, you start learning about complex numbers and all the fancy stuff. Hard, but very interesting, makes normal maths a lot easier as well.
Hope this helps!