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While I'm not sure about GCSE, I've just finished my first year of Russian from scratch at Leeds Uni. We used a book called Ruslan, which taught you everything - grammar and vocab, from scratch. It's part of a series of books, but I'm pretty sure Ruslan 1 is supposed to get you up to GCSE standard, so maybe look into that?
I did russian at GCSE. We started learning in year 10 and it was hard work to learn the basics of a language so quickly but once you'd gotten past learning the alphabet it got easier. It's a really interesting language to learn but challenging too. We had a great teacher, so that probably made a difference, and it was a small class (4 people) but we all ended up getting A*. We actually couldn't believe it, it was madness I was thinking maybe a B if I was lucky but anyway...
We used these textbooks http://www.amazon.co.uk/s/ref=nb_sb_...ya+iskra&ajr=0 They're pretty pricey and TBH they're not all that amazing but there aren't alot of books out there for people who want to learn Russian. You also need to focus on the grammar side so if you don't have a teacher (do you?) you should buy a grammar book too.
Compared to GCSE French, Russian was relatively easy in terms of the difficulty of the exams. I think they make them easier to encourage people to take the language and to take into account the fact that people usually don't start learning the language until later on.