There's actually no legal definition of 'in charge' of a vehicle. The courts are expected to use judgement as to whether you are in the vehicle, whether you are in possession of the keys or not, whether or not there appears to be any intent to take control of the vehicle.

So, in the examples you give, I'd say that :

You are with your wife, you have drunk and she hasn’t and she will drive home but you go to car with the keys whilst she is still putting her coat on. On the way home she stops at an ATM and you wait in the car with keys in ignition.

If you are in the passenger seat, then it is pretty clear that you are not 'intent on taking control of the vehicle"

What about motor or ordinary caravans? Pub landlord agrees you can park up overnight. Is it different on a campsite? Maybe if private land.

Depends on what you are doing. If you're in the drivers seat, with the engine running or even the ignition on, it certainly could be said to show 'intent'. If the public have access to the car park or caravan site, then the law applies. Private land is irrelevant.

I have on occasions gone to my car parked on the street to get something. Hotel car parks may be counted as public highway too. I am even assuming that it has to be on the public highway and not your own drive.

Again, if the public have access to an area, then the law applies. If you went to your car, got in, shut the door, and started the ignition, then you could well be judged to have 'intent' to take control of the vehicle. If, on the other hand, you opened the boot to get something out, then you are in no way attempting such 'control'.

As to your assertion that many drivers have been done for taking a nap in the back seat in the pub car park. that's a sweeping statement with very little to back it up.

The only case I can recall reading about involved one where the police were following a car that managed to 'lose' them. They then, a few minutes later, managed to find a virtually identical car in a pub car park, the person who was 'asleep' in the back claiming he'd been there for hours, in spite of the bonnet and tyres being warm. Unsurprisingly, he was convicted.

I have on occasions gone to my car parked on the street to get something. Hotel car parks may be counted as public highway too. I am even assuming that it has to be on the public highway and not your own drive.

The Lad's experience made me think about that scenario too. If we're having a family party at home I'm quite likely to park my car and/or Mrs B's on the access to our cul de sac leaving drive for vistors, including my (now deceased) Mother and others with limited mobility. If I subsequenly and well drunk go out to car to recover my coat and put ignition on 'cos I've noticed clock never got corrected for Summer Time am I in charge?

As noted in my original thread there are defences but, other than fact that there's a presumption you will drive there's not much explanation as to rationale.

Of course law could be reformed but the 'opportunity cost' of Brexit is that resource in both Westminster and Whitehall is consumed in EU exit and other stuff doesn't get a look in.