In the UK being drunk in charge of a motor vehicle is a crime. 'In charge of' includes for example being near the vehicle with the keys in your possession and certainly includes being asleep in the vehicle. If charged with this crime you are required to prove (to a balance of probabilities standard i.e. about 50:50) that you were not going to try to drive it until you were sober. NB I don't necessarily agree with the legal situation in the UK I'm merely stating the reality.