As someone who did Graphic communication in GCSE, I don’t believe there is an issue with submitting more than one sketchbook for the course, but it is best to keep those pages in the sketchbook as the person marking your course at the exam board can see what things you drew as a draft or mistake and how you reflected on those previous drawings by improving them, it can help boost your mark and your grade by showing you can reflect and improve.
I don’t know what exam board or GCSE art course you are doing though, so it’s best to ask your teacher and not to rely on my information too much. Hope that helps.