It is the law. Enforced of not, depends. Cop sitting there and not arresting in according to the law, “sounds like bullshit to me.”
We , when we arrived in the UK, were briefed on this law. Had a couple airmen speak about their experience with that.
When I was on my way to the pub with a few friends, including one who was a police officer, he spotted a drunk heading towards the car park and followed him, but he didn’t pounce until he saw the guy putting the key into the ignition before arresting him for being drunk in charge of a motor car, otherwise the guy could just have claimed he was going in to sleep it off rather than had an intention to drive drunk.