It is possible, where a vehicle is regularly driven by more than one person, that the registered keeper of the vehicle does not know who was driving at the time of the alleged incident. The registered keeper is obliged to make every effort to discover who was driving by reference to fuel receipts or any other means.
In the case of a company owned vehicle, there is a presumption that the Company know who is driving a given vehicle at any one time and that records will be kept to substantiate this. This will be especially relevant for insurance purposes in the case of an insurance claim.
If you do not know the keeper, you cannot merely refuse to complete the form. If you do this you will be liable to a fine and six points on your licence.