"The court in the Case Law of Sheldrake v DPP in 2003 gave the folowing meaning to the statutory defence.
"It is a defence for a person charged with this offence to demonstrate from the evidence an arguable case that at the time he was alleged to have committed the offence, the circumstances were such that there was no likelihood of his driving the vehicle while the proportion of alcohol in his breath blood or urine remained likely to exceed the prescribed limit."
The court later clarified
"It is not sufficient for the accused to show that, at the time of his arrest, he was so hopelessly drunk as to be incapable of driving a motor vehicle; he must show, for example, that he had handed the keys of the vehicle to someone else or that, realising that he was adversely affected by drink, he had taken a room for the night."
While he did not give the keys to someone else, he did not have the keys so why is that not a defence?