Great, I've just had a look at the marking criteria and I'm off to go get the books. I'm going to go get the Theory Workbooks for Grade 6 and 7. Not sure how fast I'll take to work through them so don't know if I should get 8 too.
I would actually recommend just getting the 6 and working through that, and then getting the 7.
You're better off buying just the Grade 6 book and the Harmony in Practice, than grade 6 and 7 workbooks if money is tight. I did Grade 6 with just the Theory workbooks and past papers, but I wish I'd got Harmony in Practice first and worked on that before buying the stuff specifically for Grade 6.
I would recommend it as it has everything you need to know in it as a reference. It's especially useful for terms and expressions, and musical instruments.