Can I retake?

I received my A level results yesterday, and, in Biology particularly, I was disappointed as I received an overall B instead of my predicted A. In 3 of the units I received an A, but in one exam I received a low C which dragged my grade down. I'm already staying at sixth form for an extra year to complete a chemistry A level which I started a year late, so I'm already a year behind for uni so that wouldn't make a difference. Would I be able to resit one unit even if I've completed the entire A level? Also, should I? because for most of the universities I hope to apply for, I need an A.