Legally you have to have permission of the landowner to even step foot on his land with your machine otherwise it counts as trespass with intent to commit robbery and you can literally get jail time and police can seize your detector and your finds collection. Usually as part of gaining the permission you would sign an agreement with the landowner to say “we split finds over the value of £50 50/50 or 70/30” or whatever you agree on. That way if the police stop you you can produce the agreement to show you’re allowed to be there and you’re protected against any funny business if you find something good and the landowner tries to claim it’s his.