GCSE physics is an extremely tough GCSE, and only the best and brightest students get the best marks on their exams. The physics of gcse is best known for its high level of mathematical content and the many equations you must remember. It's not difficult if you can rearrange equations and you're good at mathematics. They give you a giant leaf that you can take to the exam with LOTS of equations.
I got an A* in GCSE physics, but as soon as I started Level A I realized that it was worth a lot in comparison. It's a big jump from GCSE as are all A-levels, but as long as you're good with numbers and aren't particularly stupid, it's very doable. In addition to how the real reactor works, it is not particularly comprehensive, but it covers everything that is needed to start a nuclear physics course at university. I was doing some physics reviews and realized that, since September, everything I had learned could be summarized in a few lines of definitions and the equation sheet.
At GCSE I got good grades in all three Science subjects, and I had always enjoyed physics predominantly above the others, so I chose Physics at level A. For example, in additional mathematics you learn to find the equation from a nonlinear graph, but in physics you are told to ignore it. One thing to keep in mind is that the government has changed the way some levels work (including physics), so instead of taking an AS and an A2, you just have to take a few exams at the end of the two-year course.