[salad77]

> or example, You cannot arrest someone for being drunk and having their car keys in their pocket, or even being asleep in the car while drunk.

Yes you can. In the UK (since we're discussing the UK, that's important), the offense is being in charge of a motor vehicle whilst incapacitated by drink or drugs. Incapacitated is defined as being over the limit defined in law (80mg in England/Wales and 50mg in Scotland - don't cross the border after a drink!).

Mere possession of the keys or being inside the vehicle [alone] or sitting in the driving seat is treated exactly the same as driving drunk. Some pubs will offer a 'hold your keys' service where you put the keys behind the bar and come back the next day to collect the keys and car when you're below the limit for driving.