This is the most common question asked, and the qualifying phrase implied in the question is "without express permission".
All land is under the ownership of someone or some organisation, and legally, explicit permission to camp must be sought. In the vast majority of cases this is totally impractical of course, even if planned well in advance, let alone when a choice must be made in mid-backpack. Permission is very unlikely to be granted anyway since wild campers just might do some damage and will certainly do no good. On low-level routes which pass close to farms, apparently most farmers will allow a pitch for one night on request, and the Backpackers Club has a list of known regular farm pitches which is available to members.
Wherever you pitch, the landowners or their representatives have the legal right to order you to depitch and move on, and you must comply. In the mountains of the Lake District or Snowdonia this will almost certainly never happen but it just might in other areas or on low level routes, and you must be prepared to accept it. For interest, we have heard reports that somewhere on the northernmost part of the Offas Dyke Path, one local farmer has a habit of zooming up the hills on summer evenings on his quad-bike specifically to move on any ODP wild campers!. We personally know of one case where a camper was moved on in the Brecon Beacons not far from Pen y Fan, and there are reports that this area is becoming much more sensitive in this respect.
In practice, in the mountainous regions of the country, wild camping is generally accepted or at least tolerated on the 'open hill' above the farm intake walls. This is an imprecise and over generalised definition since walls and fences can often be found at all levels, but we recognise instinctively what it means when climbing the hills in any given location. You just 'know' when you have crossed this hypothetical boundary to open hillside as the land turns from working farm pasture to rough country. Even up here we try to pitch in a remote spot, away from any paths where possible for discretion (and privacy), and we have never had any problems.
There are also unwritten conventions which should come naturally to all who have a love and respect of our wild places:-
In the Dark Peak District, the access agreements expressly forbid wild camping, but in law this only makes the area the same as anywhere else in this respect. However in summer, the situation here is rather different due to the very delicate nature of the landscape and the very high fire risk. Park rangers actually walk out to the most likely areas to check for wild campers, and this is understandable when the peat is tinder dry and is basically a huge mound of fuel, and they have the constant worry of careless people starting a potentially extensive and devastating fire, such as those witnessed in recent years. The North York Moors has similar restrictions and the fire risk from camping stoves is again cited as a principal reason. Although in practice the risk from campers' stoves is very small, and certainly very much lower than that from carelessly discarded cigarettes, our advice would nevertheless be:-
** In dry summer spells, avoid wild pitching in these high-risk areas **
It isn't worth the potential hassle and ugly confrontations.
On low level routes things are a bit more tricky. There are no really remote spots here and most of it is working farmland. However with some common sense and discretion, such routes can be wild camped successfully as we have done. The most important rule here is well known amongst regular backpackers - 'pitch late, leave early'. Also implied is the universal rule that you should leave no trace of your presence wherever you pitch. In summer, agricultural work often continues well into the evening and may force a late pitch.
A particular example worth mentioning is The Ridgeway, which we have done. We heard of a first-hand report of a local farmer who said that he often sees a tent pitched on the ridge, but he doesn't mind provided it is gone early by the time he sets off for his work and that no trace is left. This is entirely unofficial and is merely a statement of the way things are in practice at that location, but it shows that all can go smoothly for everyone if people show due consideration.
Landowners generally often get flack from the walking community, sometimes quite rightly, but judging by reports of the behaviour of some walkers we have to sympathise with their dislike of the idea of wild camping. One farmer, living close to a National Trail I believe, reported that people had relieved themselves on the open ground, heaved a stone from a drystone wall and just dropped it on top. This sort of thing is indefensible and gives all backpackers a bad name.
Then there is the problem of fire risk as emphasized above. The traditional image of sitting round a glowing campfire may be appealing, but we find it hard to believe that people still light open fires in areas of risk or next to paths. We have occasionally seen the remains of such fires, with stones arranged in a circle and charred wood leaving a scar on the land, and no attempt even to tidy the site afterwards. There is no excuse for this either - use a stove, or keep well away from paths and tidy up scrupulously.
The Backpackers Club expects members to maintain the highest standards of the craft.