When I did my GCSEs a year early last year, to get an A*, you needed to get 155 and above. A was 124 and above, B 95 and above, C 65 and above, D 35 and above and E 20 and above. U= 0.
This was the Mathematics A Edexcel.
I recommend working VERY hard with past-papers, and to know the content well. CGP revision books are GOOD, but I would strongly advise getting a tutor to help you with the course content and also give you past papers for practice.
Do as many papers as you can, that's what really helps once you've learnt the content.