The post quoted above states the basic position. There are a couple of other considerations - driving onto private land for more than a set distance (which temporarily escapes me) is an offence, but one which only carries a nominal fine with no penalty points, and only if for purposes other than parking. Back into someone's driveway to turn round, an offence. Park in it, then it's simply trespass, no offence committed.
Another consideration is Drunk in Charge. There are more complete threads on here regarding that, but basically if you're in charge of a vehicle in a public place and are over the limit, you can be prosecuted. Public place, broadly speaking, is somewhere that the general public have access to, on payment of a fee or otherwise. Such as, a car park. You have a statutory defence if you weren't going to actually drive until under the limit again, but the onus would be on you to prove that you would have been sober by the time you drove off. That can involve you obtaining a toxicology report costing hundreds of pounds. If you win, you can claim the cost back from central court funds, but personally, I'd avoid the whole thing by not drinking in the first place.
Short answer, just be sensible. Parking somewhere overnight is not against the law, ergo sleeping in a parked vehicle isn't either.
but then again if your gona have one you might as well have A BIG ONE
