Near a public highway: As long as your vehicle is not causing an obstruction, there is no law against sleeping in your car overnight. On private land: in general, you should seek permission from the landowner.
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So if you're on a motorway and pull into a service station or a legal rest area, you can sleep in your car. Likewise, parking on a street or car park and sleeping is legal however you need to be aware of any restrictions. For example, most service stations will fine you if you park for longer than two hours.
Land in Britain is typically owned by someone. So, if you want to partake in a bit of 'wild camping', you'll need to get explicit permission from the landowner. If you don't, you could be in breach of the law.
If you are the driver in charge of the vehicle it is illegal to sleep in it 'on the public highway' this includes laybys! Even if you take keys out of ignition, sit/lie in a place that is obviously not the driving seat, it is still not legal.
It is legal in Scotland and some parts of Dartmoor. However, if you are respectful and use common sense, then it is generally tolerated at other places as well. A car camping awning or rooftop tent will get you a lot of unwanted attention, so I would stay away from these unless you are in a very remote area.
You can park anywhere as long as it's not forbidden. Car camping is allowed in most European countries, except Slovenia that fines heavily sleeping in the car outside of the designated (and very expensive) spots.
There must be many here who have parked overnight in a NT car park over the years, but the Trust now officially state 'No overnight parking' due to abuse/mis-use by the few who spoil it for the many. They cite litter, fires, human waste deposits, etc., as prime motivations behind their policy.
Turn Off Your Car — Never leave your car running overnight. Keeping the engine running can lead to dangerous carbon monoxide poisoning. Make a Bed in the Back Seat — If you can, fold down the rear seats of your car to create room for your bed. If you can't, sleep on the back seats.
It is not dangerous insofar as carbon monoxide poisoning is concerned if your ENGINE IS OFF and you sleep in your car with the windows closed. Unless you have a very old and leaky engine/high emissions car, in which case the window seals are probably shot, too.