DtG: My understanding of it is that if you are on a public road (including laybys, I'm unsure about public car parks) yes Mr Plod can get you as described if you have the keys, and I can't think of a practical way of not having them to hand. In fact as I understand it even if you are not under the influence you could still be done under one of the 19th century Vagrancy Acts because you don't have what is considered a fit place to spend the night.

In practice people inhabit vans in such places for days at a time, and the law usually leaves them be unless they create a nuisance, or someone perceives them as doing so and complains. This, of course, assumes you don't try it in one of the places which has local by-laws and notices which forbid you to do so.

I was never bothered myself, and none of my acquaintances have told me of having had a problem. That said as a sad old git myself, and with a rather rigid interpretation of the law, I seldom exposed myself to that particular risk.