It's difficult to determine how difficult someone will find A-Level Maths, as there's various contradictory stories of people coming out with an A* at GCSE and then a U at AS-Level, and conversely, someone coming out with a B at GCSE and then an A* at A2.
If I were to try and summarise it, the first module C1, essentially just expands upon the A/A* topics of GCSE, and then there's just a few extra things you might learn, like differentiation, integration and sequences (depending on the exam board you go through).
As a Further Maths student, I'm maybe two-thirds of the way through C2, the second module and I don't find it too bad! The style of exam questions definitely are a little unusual in comparison to GCSE, and it requires a bit more thorough understanding of what's going on to answer the questions correctly, and using the correct methods. At AS-Level at least, I wouldn't necessarily say the 'difficulty' steps up, as long as you're pretty confident on algebra and the necessary A/A* topics at GCSE, there shouldn't be too much trouble adjusting.
I can't speak for C3 or C4 though for the moment, and I'm not particularly looking forward to the Mechanics modules, but that's just me.