There is a provision within planning regulations for the temporary use of land (Part 4 of Town & Country Planning General Permitted Development Order).
This effectively lets you use a site without having to get formal planning permission for a certain number of days per year. Event organisers use it al lot for events, gymkhanas, arts festivals, local fairs and the like.
Its legal implementation is quite narrowly focussed but here are the general rules.
Firstly the land to be used for an alternative temporary use must not be part of a residential curtilage of a domestic home or building so residential gardens, car parks etc. are excluded combined with caravan sites which are dealt with by other legislation together with the displaying of an advertisement.
The land can be used for not more than 28 days in any calendar year and allows for the siting of moveable structures in connection with the use (porta-loos, stalls etc).
The period of temporary use is reduced to 14 days per year for the purposes of markets (car boots) and motor car/cycle racing. Also if the land is within a site of special scientific interest (SSI) then motor sports, clay pigeon shooting and "war games" are completely prohibited unless prior planning approval has been obtained.
Outdoor camping by use of a tent for example can be permitted under this 28 day rule on land that is not part of a residential dwelling. This use is not per person or family. It is in total for the year whoever camps on the land.
The problem for the Planners is trying to count and keep track of the days that you do the alternative use on. Therefore, it can be quite easy to stretch the days well beyond the 'formal' 28 day limit. Many people simply carry on with the alternative use until they first get caught & then continue haphazardly relying on the Planners to formerly count the number of days used per year hoping that they will miss a few.
However, be warned, that the eyes and ears of the Planners and their enforcement section are usually the disgruntled neighbours with a grudge and video cam to hand so it is often not that easy to exploit the time limits for the year.