For GCSE Maths, you just need to make sure you revise all the topics till your exams. Make a list of topics which you are confident with and those that you struggle with. This way, you will know what to revise and won't be spending a lot time revising stuff you already know. Apart from that, I would recommend doing lots of practice papers (I know there aren't that many) but what could help you is doing papers from other exam boards so you are more aware of different types of questions. Also, try to do the old spec papers. Lastly, doing topical practice questions helps a lot as you get a feel for the different types of question which can be asked.
As far as English Language is concerned, you just need to practice writing loads. There's no easy way around it. Try to set a structure to each question you do and follow that. Start by learning some vocabulary although it isn't needed unless you are aiming for a 8/9. But make sure you are critical. Analyse the extract given to you in dept and try to connect different ideas together. Linking your thoughts and showing your "train of thought" (what I call it) will show the examiner you know what you are talking about. Lastly, make sure you give layers of meaning for just one point to make because it highlights you are a person who understood the text and most examiners will be impressed.
Good luck with your GCSE.