Wikibooks have a number of OER texbooks
https://en.wikibooks.org/wiki/GCSE_Computer_Science but the pages have not been updated for over 2 1/2 years, which is a problem as specs have inevitable moved on and these are not reflected in the open textbook despite the nature of textbooks being open and editable
http://www.csfieldguide.org.nz/en/index.html
Computer Science Field Guide. Some useful videos embedded in this work but there are no answers
One a more specific level, in relation to Python training there are suitable materials such as
https://open.umn.edu/opentextbooks/textbooks/think-python-how-to-think-like-a-computer-scientist
but there are only 2 reviews (both positive of this) in 4 years. This does not necessarily indicate a well used resource
Considering the lack of powerful textbooks for the GCSE market, there are 2 other websites which are suitable: