If you are found sleeping drunk in your car, even with the engine switched off, you may be prosecuted for being "in charge of a mechanically propelled vehicle [when] unfit to do so through drink or drugs", pursuant to Section 4(2) of the Road Traffic Act 1988. Although there is a defence immediately below (stating that you're not guilty if there is no likelihood of you driving it while still in such state), I have found countless UK press articles and court texts about people being successfully prosecuted for this.
Penalty upon prosecution (DR50) is a mandatory 10 points on your licence, plus an possible jail sentence and/or 12-month driving ban.
Compare this punishment with CD40/50 - causing death by careless driving when unfit through drink/drugs, where the minimum sanction is only 3 points and the maximum 11 points, only 1 point more than the above!
Think about it - you go out, have some drinks, and realise you're unfit to drive home and don't have a reasonable alternative, except a taxi fare which could reach three figures. You could either (a) drive home regardless, risking your and other people's lives, or (b) sleep it off in your car where you left it. To me, not only is it silly that this is a crime, but it is punishing people who should be commended for taking the only sensible decision and not being tempted to drive while unfit to do so.
Also it seems ludicrous that the penalty code for actual drink driving is the same as the one for people found asleep in their car as they are fully aware that they can't drive it.
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By the way I don't care about the "well you should have thought about it beforehand" moral high-ground brigade, we are all human beings and make mistakes. You can't change the past and once in that situation, you still have a choice whether or not to drive when unfit to do so, and I don't understand why making the sensible choice can get you in just as much trouble as making the bad choice.