Wild-camping, at least in England and Wales, is strictly speaking against the law, unless done with the explicit consent of the landowner. Here’s where things immediately become murky: against the law is not the same as being a crime. In fact, under the law (at least in England and Wales) trespass is a civil offense, not a criminal offense.
What this means in practice is better explained with an example: Imagine you own a piece of land somewhere, and I trespass onto that land, for whatever reason (but I’m not breaking any other laws, and certainly not damaging anything on your land). In this scenario, your options are extremely limited. Basically, you can ask me to leave. If at that point, I humbly apologise, and start packing up, legally there is nothing further you can do. If you tried to frogmarch me off your land, I can charge you with assault, which is a crime, and if you break anything of mine, that may incur some extra charges.