Be very careful...
You are in charge of a vehicle on the public highway (or any public place, so includes pub car parks... According to the forums, a couple of Britstop users have been caught on this...) whether parked legally or not.
It's treated no differently than sitting in the seat of you car whilst drunk.
If you have access to the keys in the vehicle then you can be charged with "drunk in charge"... you don't have to be driving the van. 10 penalty points... not good.
Rgds
Mark
Advice from one of the best motoring law firms in the business, Geoffrey Miller:
Drunk in charge is a statutory offence. You are in charge if you are in the vehicle with the keys and if over the limit you are drunk, therefore guilty as charged.
However, and it is a big one.
To prove yourself not guilty you must show that you had no intentions of driving away at that moment,
Indications that would help you win your case on reasonableness:
Distance of the keys from the ignition ... I hang mine up in the wardrobe.
Seats turned round so the vehicle is not drivable at the "instant" that you are charged.
Bed made up.
Prior arrangement to stay ... e.g., having called a britstop first and confirmed you can stop overnight.
Common sense: Not having so much to drink that you would still be over the limit at the time that you would reasonably be setting off (example from Scottish police who when the limit was reduced blitzed truck stops with a breathalyser in the early hours and refused to let drivers leave until such a time had elapsed ...)
Their are no categoric assurances but the "DiC" statute is there for a purpose, to prevent a defence of "I was sleeping it off" ... (Engine still warm, sat in drivers seat, keys in the ignition ... oh! Really???) and not to entrap people reasonably camping overnight in their vehicles and having a perfectly lawful drink.
I sort GM's advice when a few scare stories did the rounds and their "no categoric assurance" summary to me was "If you show prior intention to overnight, the drivers seat is turned round, the bed made up and you demonstrating every intention to get in it if not in it already and the keys away from the ignition then we would want the defence brief"...