If what you say is true, then all you have to do is fill in the section of the NIP stating who the driver was.
Assuming for a moment though that the driver was you, I would strongly advise against trying to get out of it by claiming it was someone else. If they have any camera angle which shows you in the driver seat then suddenly you're up for perjury, not speeding - and that's a hell of a lot more serious.
BBC NEWS | England | Hampshire | Couple lied over speeding ticket