I would go down the route of using Tailored Tutors and going over everything you have done for A Levels once again. Revisit every exercise in the textbook for maths and then go onto the past papers like C1-C4. Do other exam boards questions if you need to get extra questions.
For Bio and Chem, use Quizlet for flashcards and revisit the CGP revision guides. Get your base knowledge up to where it should be. Then go through past paper questions. Make certain you have a good base. Try to explain it back and use mindmaps and draw out spider diagrams in order to get the basic facts solidifed. Then do practice papers until the exam, saving some for when you are revising in between exams.