If you were not driving the vehicle at the time of the alleged offence, then you are not guilty. The offence is one of driving the vehicle at excessive speed, not of aowning a vehicle that was being driven at excessive speed.
However, if you are the registered keeper of the vehicle, then it is your responsibility to complete the forms, notifying the authorities who was driving at the time of the offence.
Obviously, it is not enough just to say "I was not the driver". You may need to be able to back up the fact that you were not driving by providing evidemce as to what you were doing at the time of the alleged speeding offence.
