I got A*A*A* (maths, further maths, chemistry) at A level, and an A in AS biology, and a Distinction in Pre-U physics. I've never even heard about these "Seneca Premium" or "Snap Revise". I had 1 or 2 CGP books but never really used them. I feel like the key to success is to make sure you know what's on the syllabus and learn those properly, plus lots of practice. Also, the only thing I can recommend is Anki, or any other "spaced repetition" app.