Most GCSE courses for adults run over an educational year. (i.e. starting in September). There are very few, if any, that commence later on. So, for example, the free GCSE Mathematics course at Suffolk New College, near to me, is already nearly half a term in, with enrolment having taken place back in August:
https://www.suffolk.ac.uk/courses/gcse-maths
So it's possible/likely that you might have to wait until the
next academic year before you can commence your studies. Sorry :(