Nothing … a landowner has the right to not allow take off and landing of drones on their land, and can put up a sign saying so.
Overflying their land having taken off from someone elses land (with permission) is normally ok (with some exceptions e.g. flight restriction zones)
There’s a thread somewhere with a long discussion about overflying land, and some of the limitations, will have a quick look for it.
edit - I think it was this one: UK law on flying in your local park & over private land