You should be able to sit GCSE Science at a local FE college, most colleges offer this, it would probably be at a cost but not too pricey compared to what some private institutions would charge you. You could do it as an evening class. You've probably left it too late to enrol for this year but might be worth getting in contact with your local colleges to see if they run the course and if they'd be willing to accept you on to the course at this stage. Otherwise, you could wait and start next year or self-study and book to sit the exams at a private examinations centre in the summer, which for GCSE science shouldn't be too taxing.
Good luck!