I recently sat an 'equivalency' test to get my maths GCSE - and invested in 2 books to help me revise. I'll send them free to anyone who wants them just to get them to a good home!
I don't have, or know, any children old enough for these to be of use & my oldest is only 11 - they probably will have changed the whole syllabus by the time she is sitting her GCSE's! I've got the Letts 'GCSE Success' visual revision guide for maths, intermediate, and the Letts 'GCSE Success' questions and answers (also intermediate).