I’m afraid, NOX, that insurers can and do retrospectively cancel policies if they believe that there was false or misleading information provided at the time of the proposal or if material differences (e.g. convictions) have occurred since cover began and they have not been disclosed.
Chris gave some valuable advice in response to your first question concerning the details the insurers must hold of the changeover they allege was made. If what they say is untrue not only do you have mitigation to offer the court but you have a defence against the charge. You need to get to the bottom of this without delay. You need to find out precisely why cover was cancelled and ask for evidence to support this. Otherwise you might as well plead guilty and offer no mitigation. (In practice, because of the “absolute” nature of the offence there is very little mitigation available for the offence anyway and if the insurers’ version of events is true there is nothing you can say in your defence).
You must find out why cover was discontinued forthwith.