Used at both GCSE and A level; can't recommend enough. I've used Pearson, Collins, Letts, Nelson Thornes, etc., And can honestly say CGP are the best. Learn them, know them, they'll do you good. If you want top top UMS one resource won't be enough so I'd recommend checking out a variety -- but definitely get CGP. The student books for A level are superb and the revision guides are good too.
They're clear, concise and use plain English. The nelson thornes books my school uses just don't make sense, and aren't worth twice the price.
Best of luck! X