Use Cognito and Seneca for the science GCSEs. They're free online revision sites that can provide a good overall understanding, and the CGP books will give you questions and help you both fill in any gaps and consolidate what you've learnt.
Bear in mind that if you are taking the sciences as a private candidate, you want IGCSEs not GCSEs, otherwise you'll be faced with the problem of having nobody to sign off on the practicals involved in the GCSEs.
Thanks so much for these suggestions! I will have a look at all of them. I hadn't actually thought of how the practical areas would work if I am teaching myself from the internet/books.