Some schools offer a free resit whilst some range from £30 to £100 per exam subject. I believe you can enter GCSE subjects this summer (May/June) in 2024, where other people in Y11 will also be taking their exams. You can enrol to resit your GCSEs at a local school or college.
(reminder: I did research online! So, if I'm wrong, please correct me!)
When? May/June 2024 or 2025 How? find a centre willing to enter you and pay the fees. GCSE science exams in England are only offered in the May/June exam window, The deadline for entries in summer 2024 has already passed. You could still be entered but many centres won't do it and the fees would be high if you can find a centre that will. Your school exams officer or head of science could clarify if they would be prepared to enter you as a private candidate, If not then you have to search for a centre that would. The JCQ web site can help with locating possible centres https://www.jcq.org.uk/private-candidates/