hiya, if you google astronomy gcse textbook as far as i'm aware there's only one but thats what i used to make flashcards off and it was pretty good (not amazing, but the best you can get really as it's the only one out there)
i'd also recommend astronomy gcse help on youtube, and just google astronomy GCSE onto youtube as there's multiple diff channels that will help. personally, i preferred to use the textbook but my friend liked the videos a lot more to make flashcards on and we got the same grade so it's just ab what works best for you - try them both out and then decide.