There were scans of the earlier textbooks on the internet, unofficially uploaded. There is enough on the Cambridge Latin Course site to be getting on with and sites where you can buy things without using pirated books.
Not a Latin teacher, but I think it depends on which board. The Cambridge Latin Course does work towards GCSE under one board, I think it's WJEC? But they have a double qualification (language & literature?) so not sure how the CLC books fit in to the syllabus.
Dd would like to do GCSE Latin, grateful if someone could advise a good study book to use at home. There seem to be very few revision guides available compared to other subjects. I know for French & German the school will be using the AQA exam board, not sure if this is likely to be the same for Latin. The book she uses at school are Camebridge. Unfortunately they did not keep it over the summer hols,