Wild camping.. not legal, but its civil rather than criminal unless you actually damage something.
In a car, really, not that practical - the places you'd be able to discreetly camp, you wouldn't discreetly park a car anywhere near and its going to be a long old walk from where you can park a car.
On a bicycle, it is much more do-able and I know a couple of people who do this for two or three weeks every summer and are very discreet, very self contained and have only been shouted at a couple of times (neither occasion by a land owner but by dog walkers who thought they were homeless/rough sleepers).
You run the risk of - being shouted at, animals (cows are NOSY sods and horses are too both can be very dangerous even when just mucking about let alone frightened or feeling territorial) er and the general dangers of being in an isolated place on your own..
Whoever said the land owner is liable for any injury/damage etc to a trespasser/their property..
Not strictly true, the Occupiers Liability act was challenged, see Tomlinson v Congleton BC - the short version of that is, if the trespasser should have reasonably predicted the danger - ie, a field may contain large nosey animals that kick people/bite people, the occupier is NOT liable.
If however the trespasser could NOT have reasonably know the dangers - ie, your land has a concealed 100ft mineshaft or is filled with land mines and theres no warning signs indicating this and the trespassers gets his legs blown off or falls down a hole - the occupier IS liable.
S'all about reasonable, common sense, so you are not going to be successfully sued by the wild camper who gets trampled by your nosey horses.