I wouldn’t bother with the online courses, I bought a maths GCSE one and barely used it - I printed the spec out & used the maths gcse tutor videos and got a 7 (I left school in 2008 with a C) - after that experience with the online courses seeming useless, I printed the spec for igcse edexcel science (if you do GCSE, the exam board requires you to go into a school/college to do practicals - which aren’t weighted in your grade, just part of the course) I used science with hazel YouTube videos & got a 9 in biology. IGCSE is internationally accepted - unis accept them equally to GCSE.
I think the online providers are somewhat good for structure, but if you buy the CGP books, that gives you a guide of the order of topics. You just pay for the exams with the exam board, but there are certain cut off times for entering; you can check this info online - I used tutors and exams in Coventry to sit my exams.
Hope this helps!