Hi - I'm in Y12 and so have been studying History at A-level for just over a month now. It is tough and in much more detail than GCSE, but if you enjoy it, it's very rewarding and really helps you think in different ways. I'm doing it with science subjects, so it's quite a contrast to my other subjects and is certainly a lot more work than say, Maths or Chemistry, at least at the moment.
It's a very rigorous A-level and is well looked upon by universities, and equips you with lots of transferable skills for the future. If you enjoy History at GCSE and are reasonably good at it, it's likely you'll enjoy it at A-Level too, as long as you are prepared to put in a lot of extra study.