I've been in near enough the exact same situation as you. Currently at college I am in my first year, studying biology, mathematics and geography.
I had only got a 7 in maths at GCSE and have even had a year out of mathematics all together, going into the course my teacher did warn me that it's going to be hard, which it can be at times. But if you really do want to study it and you enjoy it, like myself, you will definitely succeed. She worried I wasn't going to make it but recently had a test and I was second highest scorer and I'm working ahead of the class I'm in currently, and I don't revise it consistently I do the prep-work (homework basically) she gives us each week for about 3 hours a week and that's it. So go for it, you won't regret it even if it is difficult at times, your definitely capable.
In terms of your other a-levels, very good choice and very ranged. I wish I had done chemistry, as I wish to maybe go onto foundation medicine or biomedical science. Don't put to much stress on yourself about a-levels though at the moment, revise and get your GCSEs and in the first 2 weeks of college they usually give you the option to change if it doesn't appeal to you.
Good luck with your GCSEs!