The UK looks like one island on the map, but access law is devolved, and each nation has its own settlement with landowners, farmers and the hills. A bivvy that is perfectly lawful above Callander can be trespass in Cumbria, and a pitch in Eryri that has gone unchallenged for years is still, strictly, a matter of the landowner's goodwill. Learning the differences takes ten minutes of reading and will keep you on warm terms with the people whose land you are sleeping on.
Scotland is the outlier and the envy of walkers south of the border. The Land Reform (Scotland) Act 2003 gives a statutory right of responsible access to most land and inland water, and the Scottish Outdoor Access Code is explicit that this includes wild camping. You can pitch a lightweight tent on unenclosed hill ground and across most open country.
The conditions matter as much as the right:
The golden habit is to camp above the highest enclosed land — above the top wall, fence or dyke — where you are on open hill and clear of livestock, machinery and the farmer's morning rounds.
England has no general right to wild camp. The Countryside and Rights of Way Act 2000 opened vast areas of mountain, moor, heath and down for walking, but camping is not one of the permitted activities on that access land. In practice, this means you need the landowner's permission.
Dartmoor has long been the celebrated exception, but its position has been unsettled by litigation in recent years. Camping on parts of the commons was widely tolerated and treated as permitted for decades; a legal challenge narrowed that position, and the current arrangement depends on a permissive scheme overseen by the national park authority and landowners willing to take part. Rather than relying on rumour, check the authority's current guidance before you go, and ask if in doubt.
Elsewhere in England, the openings worth knowing are:
Wales works on the same principle as England. Open access land grants walking rights; it does not grant the right to sleep there. Pitches in Eryri and the Brecon Beacons may be quietly tolerated in remote spots, but tolerance is not law, and a warden who asks you to move is within their rights.
Welsh access reform has been discussed for years, and low-impact campsite provision has grown steadily, but for now the reliable routes are:
Northern Ireland has no general right to roam. Public access is largely limited to public rights of way, permissive paths, specific access agreements and land managed by public bodies. Wild camping outside organised sites effectively requires the landowner's permission.
Practical options are limited but real:
Whatever the legal position, wild camping is a privilege earned by behaviour as much as an entitlement, and the walker who leaves no trace is the one who keeps access open for everyone who follows.
Read the access code for the nation you are walking in, ask when unsure, and treat every pitch as a visit to someone's livelihood. Do that, and the hills will keep their welcome generous.