Seneca is a free resource - not so good on overall scope of the curriculum, but good on the individual questions if that makes sense, and is divided into exam boards.
A decent revision book for the gcse board in question
look up YouTube tutorials - some are very good
but the biggest thing for us was exam technique, and printing out loads of past papers AND their marking papers.
It really helped Ds understand how marks were allocated (eg a 4 mark question wouldn’t achieve 4 marks for just the right answer (one mark only): you’d get one mark for stating Pythagoras, one mark for stating the theorem, one mark for substituting numbers into the theorem, one mark for the right answer- so you could get 3 marks even if the final answer was wrong ). The past papers also give a good feel for how the paper is made up, of the 8 mark, one mark etc questions.
This allows them to develop a bit of a strategy - moving quickly past the “too difficult” questions, marching through those they can do, then going back and “mark scraping” even partial answers to the tricky ones.