There is a huge amount on .gov/UK regarding pemissive rights of way but in essence this sums it up :
Permissive paths, sometimes referred to as concessionary paths, are not public rights of way and the public do not have a legal right to use them. They can play an important role in improving public access to the countryside. Providing a connection between existing public rights of way where gaps currently exist in the network. As permissive paths are not public rights of way, landowners may impose conditions on their use. This can include restrictions on when the public can use them and how the paths are used (e.g. walkers allowed, but not cyclists or horse-riders).
Creating a permissive route
There are two main ways of establishing a permissive path either:
through a formal written agreement between the Council and the owner of the land, or
by the owner of land granting consent in a less formal arrangement
Usually, a formal agreement between the landowner and local authority is in place. This ensures the authority will address problems encountered by users with the landowner. The agreement can include: