Where you can leave the path, and what you still cannot do
A footpath is a line. Step off it into the field and you are trespassing, however empty the field looks. Open access land is the exception: ground you can cross in any direction you like.
Government guidance for England describes it as mountains, moors, heaths and downs in private ownership, common land registered with the council, and some of the land beside the King Charles III England Coast Path. It names the right twice over: right to roam, freedom to roam.
Walking, running, watching wildlife, climbing. That is the list.
Cycling, horse riding, camping, water sports, driving anything other than a mobility scooter or a powered wheelchair, and bringing an animal that is not a dog. Camping is the one people get wrong: access land gives you the day and not the night. Bikes and horses still get through where a bridleway or byway crosses the land, where the owner allows it, or where a local tradition or right of access is already there.
On a lead of two metres or shorter between 1 March and 31 July, to protect ground-nesting birds, and on a lead around livestock at any time of year. Next to the coast path the wording is close control. There can be other local or seasonal restrictions, and those do not apply to public rights of way or to assistance dogs.
Inside the mapped area sit pieces that stay private, and the map does not say so. Houses and other buildings with the ground they stand on, land growing crops, building sites, parks and gardens, golf courses and racecourses, railways and tramways, working quarries. To get across one of those you are back to a right of way.
Source: Rights of way and accessing land: use your right to roam (GOV.UK).