CGP are mostly great for sciences, as long as you revise with each individual specification at hand (print it out!!!). I've seen them leave out things, add in unnecessary things, and even make horrible mistakes for key things like say, electrolysis for chemistry? So using the specification and online resources like freesciencelessons is super important. But other than that, they're great. Buying textbooks for the sciences is way too extra - just use CGP and your specification. You're aiming for an 8, so I assume you don't need that much help anyway.
I wouldn't recommend buying a revision guide for a subject like maths. Unless you genuinely believe that revising for maths like you would an essay subject (i.e making notes) would help you, then you shouldn't buy revision guides, only workbooks. Maths is all about practice, and really you can find thousands of past papers, specimens, and there are resources online like MathsWatch, and other sites. Buying things for maths sounds like a waste of money, unless you reaaally think you need workbooks, or you went through every resource online.
CGP is generally good for humanities like english and geography, but like any other subject, you need to practice too.
I'd especially recommend CGP for Spanish - I used CGP for my french exams, which i just took last week. They were a great help with grammar, topic vocab and general vocab, along with a handy glossary of words your exam board expects you to know at the back. I took AQA though, for French, but i doubt it'll be very different.