Schools seem to differ on whether they make recommendations for specific revision books, and subscribe to different online study websites - there seem to a lot of subscription options for maths, and for science like tassomai and mygcsescience.
We’ve been gradually making note of teacher YouTube channels / free websites for the subjects DS have chosen, so will list them here in the hope they help. Some may be more helpful for certain exam boards. Some I found just by searching on YouTube for e.g. “gcse geography aqa”.
Physicsmathstutors - sciences, geography, some others I think
Savemyexams - sciences, maths, and past paper links for other subjects
Freesciencelessons - sciences
Seneca - just about everything, but I’ve heard you need to top it up with other content
Primrose kitten (YouTube) - sciences, geography
Mr B (YouTube) - geography
Craig’n’Dave (YT) - computer science
MrBrownCS (YT) - Computer science
Bizconsesh (YT) - Business
Guiglit (YT) - English
Mr Salles (YT) - English
Mr Bruff (YT) - English
CorbettMaths (YT) - maths (his website also has question papers)
Stacey Reay (YT) - English
Takingthebiz (YT) - Business
News in Slow Spanish - Spanish listening practice
There doesn’t seem to be much that’s GCSE-specific for languages, I guess because of Duolingo and Memrise.
I’m sure there’s plenty more I’ve missed!