I didn't feel a jump from GCSE's to A-levels. But personally I found a massive jump into year 13 that was related to me just taking a while to get my head around magnetic and electric fields. But how I found it could be more related to my mental state or the amount of support I had at both stages.
Anyways, I managed an A*.
And although I think its really tough to get an A*, its not at all hard to get a high A, as long as you make sure you leave 80-90% of your lessons with a good understanding of what you were taught. And if that's not possible, then at the very least leave that day or that week understanding it. That applies for every topic, really. Things pile up and you'll never actually come back to that misunderstood topic, not until its exam time or too late.
Don't worry about people saying it's one of the hardest A-levels.
It's like with maths, the people who are naturally good at it take it and don't find too much difficulty, and everyone else stays miles away, and from that distance even a simple quadratic question looks insanely complicated.
Personally, I'd say English is one of the hardest A-levels, that their requirement to learn tens of hundreds of quotes, and wider reading, and essay writing, are skills I wish I had but am just not capable of doing to a good enough standard. Believe it or not I still really wish I was capable of essay writing, and could take a humanities degree at university. But my safe option, surprisingly, is physics.
And finally, although considered difficult, physics is very self contained. You don't need a single bit of knowledge outside of your textbook, and that can be a blessing if you are a student with the 'bare minimum' mentality. I certainly was at A-levels, and I still got good grades so...
I hope my experience and opinions helps in some kind of way