There's a difference between drink driving and drunk in charge of a vehicle. If you are over the limit and in the car where there is a realistic potential that you could drive that car whilst under the influence then you commit the offence.
In Shackleton's case he was over the legal limit

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Given that he was allegedly in a country park where there is nowhere to go it's highly likely that he would've driven the car whilst over the limit. It would be for him to convince the court and prove that he either intended for someone else to pick him uo or get an Uber home. Even if he said he was going to sleep in the car until he sobered up he would still commit the offence.
In a nutshell, he's probably screwed on this one. Most plead not guilty to keep their driving licence for longer as you don't get banned until you're found guilty in court. So most plead not guilty at the initial hearing then plead guilty when the trial goes ahead months later.