Difficult one.
Perhaps if you have a lockable glove box or something and the keys were in there? The boys in blue would at least be able to see you have made every effort to demonstrate your not going to be driving. However, there have been cases where the Police have been a bit over zealous recently such as this one:
http://www.telegraph.co.uk/news/newstopics...d-at-wheel.html
When you can get 3 points for eating a snack, smoking a fag or turning up the volume on the stereo, gawd knows what they could do if you have a couple of sherberts and get one with a bee in his bonnet.
Technically, even if you're parked up in a Private caravan pitch and have had a couple of drinks, you can still be prosecuted. Scarey isn't it!!