Resitting GCSEs

I know I don't know my results yet and anything could happen, but I'm fairly sure I haven't done as well as I would have liked on my GCSEs. I was wondering if it would be possible to resit a few next year, but without having to take extra classes in it along side A levels and just resit an exam. Also, would this be possible for subjects like geography or can you only resit maths and English? I don't think I've failed my GCSEs but I definitely don't think I've achieved the grades I want, if it turns out I do get a B or C in geography and English, but I want to resit them to get a higher grade will I have to take another year of these classes and is this allowed? Can I just sit the exam at the end of the next year and do self studying?