I was kind of ignorant of tose soft alevels and facitating subjects that the top unis make a fuss about. If you have set your sights on those unis then avoid the soft subjects.
When the exam period comes, you'll regret having gone out partying every weekend, no matter how much fun it was at the time
Also don't pick subjects you have no interest in just because they look good on your uni application, you'll never be willing to put the time in to get the best grades if you dislike the subject (especially physics)
I wish I could have been told a couple of things, sorry about the wall of text
If you're studying science subjects, you can no longer get away with coming home and bumming around on TSR/Facebook etc all day.
If you do an hour's revision for each hour taught everyday, you won't have to do intense revision around exam time.
You can still get away with cramming around exam time for the first year but it's really stressful and won't work for your second year because at A2 the exams are based on applying your knowledge in "unfamiliar context." Memorising the content of the book won't be enough anymore.
Your teachers won't spoon-feed you during lessons anymore. The difference between the A grade students and the C grade students in my year is that the A grade students go over the next lesson's topic at home before the lesson that way it's like your lessons are just revision classes. Whereas the C grade students don't know what's going on until about a week before exam time.
If you want to get a head start and start learning during the summer then go ahead. The only kids in my year that did this are the only two that got straight As in their January modules.
And don't go thinking you won't be able to deal with the 'huge jump' between GCSEs and A Level because you're more than capable. You just have to get used to pulling your own weight instead of relying on your teachers to teach you everything.
EDIT: wow, sorry, didn't realise this had caused so much distress...I only took Chemistry because I thought it would look good whatever I applied for and thought I could do it well, clearly I was wrong and ended up doing Politics and Philosophy with a B in Chemistry A Level so, for me, it was not worth it.
Don't take 5. It's not impossible to do, it just doesn't let you focus on a subject that you want to do at uni, it doesn't let you do background reading, learn stuff beyond the curriculum etc.
Don't expect people to actually be interested in the subjects they're studying, or that the lessons will be any better than GCSE. (The people in the year above who told me that must secretly go to a different school...)