A tragic and alarming case is reported by the Guardian and other national newspapers, which describes how the driving exhibited by 75 year old Shelagh Robertson in January 2021 led another vehicle to be diverted from its course and to crash into a mother and her 5 month old son. The baby died of his injuries. The case came to court in Cambridge recently and Mrs Robertson was acquitted on the grounds of insanity: Woman cleared over baby boy’s death due to then-undiagnosed dementia | UK news | The Guardian
There are more details in a local Cambridge newspaper, but I have not found a full account of proceedings. Jurors deliberate in trial of pensioner accused of causing death by careless driving of baby Louis Thorold in Waterbeach (cambridgeindependent.co.uk)
The essence is that Mrs Robertson has been found to have dementia, probably Alzheimer’s disease. This had not come to medical attention before the accident occurred, so that she was not aware of the condition. The diagnosis has been confirmed by clinical assessment and MRI scan before the trial. Professor Adam Zeman observed that her condition was such at the time that he examined her that he would advise her that she must stop driving.
Evidence from a friend suggests that changes were apparent in Mrs Robertson’s behaviour since 2016. The friend last spent a journey in a car driven by Mrs Robertson in 2018 and felt unsafe.
Driving with dementia or Mild Cognitive Impairment: Consensus Guidelines for Clinicians (2018)
Judge Mark Bishop directed the jury to consider Mrs Robertson’s status against the M’Nauhten rules, which remain the reference for the legal concept of insanity in this country. They were formulated in 1843 an related to a case where the accused acted under the influence of delusions. M'Naghten's case  UKHL J16 (19 June 1843) (bailii.org)
Insanity is confirmed and can be used as a legal defence if:
The individual has a defect of reason.
This defect of reason is caused by a disease of the mind.
The defect of reason is such that the individual did not know what they were doing or, if they did know, did not know that it was wrong.