Ethel, as she usually is, is quite right.
Registered keepers (RKs) of vehicles are expected to be able to provide details of who was driving their vehicle at any particular time and the prosecuting authorities most certainly do not simply accept that they have lost out if they say they cannot.
In the case of camera detected speeding offences RKs can expect (in normal circumstances) to receive a Notice of Intended Prosecution (NIP) within 14 days of the alleged offence. This is an important provision because it means that usually they do not have to recall events of much more than a fortnight ago.
The idea that the photograph should assist them to do this is unfounded. The photograph is there to prove that the vehicle was speeding, not to identify the driver. If you (as the RK) cannot provide details of the driver you will be summonsed under Section 172 of the Road Traffic Act. This says (briefly) that a registered keeper is guilty of an offence if he does not provide the said details when required to do so. If you want to argue the case along the lines you suggest (i.e. that you cannot recall who was driving) you will have to attend court and plead not guilty. The matter of speeding (whoever was driving) will, by then, have been discontinued.
If the prosecution can show that you were sent the required NIP and that you did not respond in time, or responded with insufficient information, the burden of proof then shifts to you to demonstrate to the court that you were unable to recall who was driving. It is a difficult defence, and, as Ethel says, is more often than not unsuccessful. However, the �celebrity� couple Christine and Neil Hamilton successfully used it in November 2003.
From 24th September the minimum penalty for those convicted of Section 172 offences increased from three to six penalty points.