All land in England & Wales is owned by the Crown, ever since a certain William the B******* popped over in 1066 and conquered England. Wales was added later by his heirs. Scotland, as ever, is different.
Good King Bill then divided England up amongst his barons, keeping ownership but giving them a tenancy on the land. The barons continued this tenancy and sub-tenancy down to the level of us plebs. That's why you don't own the land your house stands on, you own the Freehold (you don't pay rent to anyone) or the Leasehold (you do pay rent to someone).
That's why the Crown finds it so easy to acquire land for various purposes; they already own it.
Edit: Footpaths and public highways - you have a legal right to "pass" and "re-pass", you can be done for loitering! You don't have a right to picnic, or watch racehorses (famous legal case, the name escapes me). You probably don't have the right to place plastic boxes, or indeed to grub about in the hedge looking for them! The answer is to be discreet.
Regards,
Neil