No. When it comes to parking, you can’t park anywhere you fancy. Even if you park in a designated car park, there might be rules on overnight stays.
You should also bear in mind that most of the land in Britain (particularly in England and Wales) is owned either by a person or an organisation. In other words, if you decide to park anywhere without explicit permission, it could mean you’re trespassing. For further info, see our guide to parking laws for motorhome and campervan owners.