It's very difficult to give a straightforward answer, the problem is that many bits of legislation may apply, including the Caravan Sites & Control of Development Act, and recent legislation covering gypsies and travellers. As a motorhome or campervan is classed in law as a 'caravan', sleeping in it may constitute 'camping'.

The interpretation of the law is that you cannot 'camp' on land in England & Wales (the law may be different in Scotland and N. Ireland) without the permission of the land owner. As far as the highway is concerned (this includes lay-bys) the land owner is normally the local authority.

However, whether you are moved on or not will depend on the attitude of the local authority or the local police. Our opinion (and it is only an opinion) is that you are unlikely to be moved on if: The vehicle is not causing an obstruction (but remember that there is no 'right' to park any vehicle on the road); there is no specific local by-law prohibiting overnight parking or sleeping in vehicles; you are not in the same place for more than a day or two.