I personally don't like CGP when teaching students.
It has scope, but really lacks in content. Use the Edexcel book, and just do loads of past papers. The content of biology is not hard at all. It is all about few basic concepts and applying your logic to it. If you encounter a question you can't do, consult the book, and the wonderful world of the internet if the book doesn't help. CGP tends to have slightly muddled up explanations, simply due to it being rather "bullet-pointed", shall we say.
If you're on the lower side of grades atm, the CGP will help. But for students who are at B/A/A*, it won't help you get those extra few marks which aren't simple memory tests.