I haven't done IT at A-level, only GCSE, but speaking to some mates who did keep going to A-level, many will tell you that it's not hard. It just requires a lot of time.
I remember when I did it at GCSE, I hated it. I had to do so much coursework, it took ages, I had to stay behind after school sometimes just to stay on top of it. It wasn't very hard, it was just long. So much so that I didn't bother staying behind long enough to get the coursework (60% of the GCSE) to the A*, I pushed for the steady A. The exam though was just so easy. They're all so similar, I got 100 ums and so got an A* overall, but it was literally basic exam technique and the same recycled questions from past papers.
When you speak to people who do it at A-level, they might tell you it makes you hate IT, it feels kind of useless at a-level, it's a lot better at degree level (computer science and other it courses). If you're thinking about taking it at a-level, remember it's not that's it's difficult to get a A/B, it's the time. So that's why they've black listed it, they don't respect is as much as the traditional subjects.