It's sort of mixed~ A lot of people say IB is a lot harder than the A levels (coming from people who actually do them) but I know a real life friend who went from IB to A levels and she believes that they are both equally tough. As long as you go for either one that's fine.
In IB you have to do a lot of work though. Managing clubs, starting your own activity, doing a four thousand word essay with lots of research into it. The exams are extremely tough in my opinion, but that's maybe because I'm just dumb (LOL jk jk). I've studied for these mocks and I'm really suffering though, if I tried to "Wing it" like my IGCSEs, chances are I'll get all Es and Fs LOL. Don't do that, whatever you do. I'm really serious, it's impossible. Don't think you can wing it like your IGCSEs because you cannot, unless you've been studying since day 1 or have an invisible iPhone or have a Mirai Nikki (Anime reference...anyone? No?)
There were people in my school who's done really well in IB though, so well that they're got scholarships everywhere. One girl even got a scholarship to Yale. Amazing.
If you like work and if you're those people who would like to be the very best like no one ever was and if you're extremely confident you're an A+ student, choose the IB. Believe me, the IB stuff are nothing like the IGCSEs or the O levels. If you're a person who just wants to fit into a good University and manage to squeeze into one of those top universities, go for A levels.