I paid for one to one theory lessons through school for my DD which was an expensive way to do it but she's just got a really good mark (not that it matters - you only need to pass!). I feel it was worth it as she definitely wants to do music GCSE and the teacher has given her a strong set of skills. You can do group lessons which are cheaper. For DIY someone mentioned mymusictheory.com, which seems to have quite a lot of free material as well as stuff you have to pay for.
The hardest bits of the exam are transposition and composition. I'm not sure how easy these are to DIY. They also have to know vast lists of Italian, French and German musical terms which just takes time and application.
As for workbooks, my dd worked through the ABRSM grade 4 and 5 Music Theory in Practice books, with the teacher, then she used the 2013 and 2014 past papers (bought from the ABRSM website online). I also have a book called The AB Guide to Music Theory Part 1 which she found useful for reference.
Thanks. That's very helpful. Do you mind me asking how musical you are? I'm only asking because I'm wondering how much help I am going to be able to give them. It sounds different from when I did my theory (in the 80s) but I can transpose easily enough. I did grade 8 on my instrument and music O' level but not sure if that will make up for my lack of teaching skills. ..