They're usually only bothered about the highest, but I always take all GCSE, A-Level and Degree certificates with me to interview just in case.
On a CV, words like "8 GCSE's (4 As, 4 Bs)" should suffice. Any more looks like padding. Depends whether you've got something to hide I guess.... e.g. "6 GCSE's (C and above)" looks better than "8 GCSE's (6 Cs, a D and an F)".