You have to realize that everyone needs time to adapt to a new educational system, including you. It does not matter what type of grades you got at GSCE or how intelligent you are, virtually everyone starts from scratch.
Be prepared for your grades to plummet during the first few months, because that's completely natural! There are a lot of formalities that need to be taken into account in all subjects, like perfect notation in maths, critical thinking in History etc, before you will get the top grades. Simply getting all the right answers won't necessarily get you a 7, if you haven't shown your reasoning.
Also, you have to learn to not compare yourself to others. Just because a certain person you know didn't get a high IB score, does not mean at all that you can't get it.
Bottom line: Don't freak out if you don't get the grades you expected at first. Keep working and you'll get there.