Can you retake GCSES?

I am currently doing my summer GCSE's e.g English, the last modules of science, and I feel I have not revised sufficiently, and have missed out whole chunks in my exams. I am now wondering if it is possible to re do these exams (seemingly biology and english-two subjects I am supposed to be good at but which i have really messed up) next year in sixth form. Is this possible? Are there fees? and when would I redo them? thankyou.