I might suggest doing some A-Level papers and seeing what areas you're falling short in - then put in the work to iron them out. I would advise against doing the OU ones as they do not provide the mark schemes, however they are good to get a sense of what questions will be on the actual exams.
If you want to go the textbook route, some good textbooks for mathematics which cover mostly everything and can be found relatively cheap 2nd hand are: K.A. Stroud - Engineering Mathematics, Anthony Croft & Robert Davison - Foundation mathematics and John Bird - Engineering maths.