seneca learning is good for all subjects.
mrbruff and mr salles were particularly helpful if you were doing aqa english lit and lang, though their advice may be applicable to other exam boards also.
freesciencelessons was a good yt channel for science; explained concepts well.
khan academy's good at explaining maths stuff. resources i found useful for maths included hegartymaths, mathsgenie and corbettmaths. lots of practice questions for the topics and good yt videos too if you get stuck.
shmoop, sparknotes, cliffnotes, gradesaver and genius are some very useful websites for english.