I think that the jump from GCSE to A levels is particularly difficult in science, maths and languages, from what I've learned over the years. Unfortunately, a few years ago my daughter's school chose to do the 21st century science syllabus and this was inadequate preparation for further study. She has done very well at A level, despite this, although still finds chemistry rather challenging at times. Many of her peers got A* at GCSE and really found it quite tough at AS and A2. The science teachers actually asked parents to complain to the head teacher about the choice of syllabus, as they weren't happy to be using it either! I suppose it was all about league tables, and the head didn't care about A levels, just about getting as many A-Cs as possible. In the same way, schools still 'shop around' to find what's perceived as easier exam boards for A levels.
If you enjoy science and think they will be useful for your future, then go for it. I think it's a great idea to read ahead over the summer break. Good luck!