“Well if he really didn't remember who was driving, how can he be guilty of not saying who was?”
Need to be a bit careful. This is a matter covered by Section 172 of the Road Traffic Act. This says that the Registered Keeper of a vehicle (or sometimes others, but that does not concern us here) must, when asked to do so by the police, provide the details of the driver of the vehicle at the time of an alleged offence. To fail to do so is an offence. There is a statutory defence to the charge which says (briefly) that a person shall not be guilty of the offence if he did not know who was driving and he could not, using reasonable diligence, find out who was. “Reasonable diligence” is not defined and if the defence is put forward it is a matter for the court to decide whether the diligence exercised is sufficient for the defence to succeed.
Normally, simply pitching up and saying “I can’t remember who was driving” (as Lord Howard effectively did) will not satisfy the “reasonable diligence” test. If it did everybody would do it and nobody would be convicted of a motoring offence where they were not stopped by the police at the time. So the answer to Hopkirk’s question “…if he really didn't remember who was driving, how can he be guilty of not saying who was?” is that he can and most people would. However, Lord Howard’s appeal went a little deeper than that and hinged on a point of law concerning the format of the paperwork which was sent to him.