I'd recommend sticking with A levels unless you want to study abroad. I did the IB myself and it was a LOT of work and left quite a few of us a bit burnt out - it is 7 subjects after all. It does turn you into an intellectually well-rounded person with a strong work ethic, but it doesn't increase your chances of getting into uni at all compared to A levels, so you might as well spare yourself the trouble! Unis also do tend to give higher conditions for IB students - they give you conditions for your higher level subects as well as your overall score. This means that if you mess up one of your standard level subjects (which will probably be completely unrelated to the course anyway), you might miss your offer. Not necessarily the case, but something to think about.
If you do A levels, I'd skip History and English too - if you want to get into good unis for maths, you will have to show ability in more than one science subject. There's a lot of other things you can do with maths apart from a straight Maths degree - business, finance, physics, management etc, so you won't be limiting your options.