True, but if you consumer alcohol while parked up in the lay bye, is there a possibility that you could be charged with being over the limit as the car keys will be in your possession. Same may apply to a pub car park?
From someone who enforced the drink driving laws I can say that it is in no way that simple. First up the offence is being in charge, which is a very long way from actually driving. Below is from the CPS website.
In relation to being in charge of a motor vehicle, a person is not guilty if they prove that the circumstances were such that there was no likelihood of them driving the vehicle whilst the proportion of alcohol in their breath, blood or urine exceeded the prescribed limit.
So basically the prosecution woukd need to prove, beyond all reasonable doubt, there was a strong likelihood of the person driving. If your in a MH or caravan, in your PJ's or similar, with the car keys tucked away in a cupboard etc (rather than in the ignition) that would be somewhat difficult to prove. The CPS would be
very reluctant to authorise a prosecution unless the driver was sat say in the drivers seat whilst in physical possession of the keys, if they were in the glove box etc then they would almost certainly not authorise a prosecution. It
could be argued that a campsite is still a "public place" so what if you have a drink or two there??
I cannot recall a single incident such as your scenario during my 30 years service. But, before anyone leaps in, it has probably happened somewhere, but the circumstances of that particular case would need to be very carefully looked at in their entirety.