Here are a few resources that could be helpful for exam preparation! These are resources which I have personally used over the years. (I am not affiliated with any of these or claim to have any ownership of them).
This Page will be updated now and then, as I stumble on new resources
Isaac Computer Science: https://isaaccomputerscience.org/
Ada Computer Science: https://adacomputerscience.org/
Royal Society of Chemistry: https://edu.rsc.org/student
Freesciencelessons (GCSE & A Level Sciences)
Mr Bulmer's Learning Zone (IGCSE & A Level Computer Science)
Cognito (For Sciences):
Mario's Math Tutoring
IGCSE Online (For multiple subjects):
IGCSE Computer Science
Science Shorts:
Alt Academy (For multiple O & A Level Subjects:
MrBrownCS
James Gan (For CS & Physics):
Slate Pakistan (For O & A level subjects):
GCE Guide: https://papers.gceguide.cc/
PapaCambridge: https://pastpapers.papacambridge.com/
Rove Papers:
Flocus - A great timer: flocus.com
CIE Website: cie.org.uk/alevel or cie.org.uk/olevel or cie.org.uk/igcse
(Pro tip, you can type cie.org.uk/(your subject code) and it will redirect you to the subject's page on the CIE Website. So for example cie.org.uk/2210 would take you to the O Level Computer Science Page)