The police are a little hamstrung there.
It can't be attempted murder, as there was doubtless no one there when the scumbags placed it. The points to prove for the offence are not met.
You can't recklessly murder someone, or attempt to do so, and until someone get hurt there is no assault to prosecute.
If someone did actually die you could have a manslaughter, but you can't have attempted manslaughter, so that dog doesn't bark until there is a corpse.
We're looking at criminal damage to the fence, or may be public nuisance if its a brave bobby and a lenient custody sergeant.
It is an offence to deposit any item on or above a highway that could endanger a user of that highway, but I'm unsure of the status of that track and it seems doubtful the definition of a highway would apply. No ball game there.
And even if there were a crime, there clearly aren't any witnesses and the scumbags we very cunning in not leaving a written confession with a map to their home aderess at the scene.
So the law is a bit J. Arthur on that one until after someone has been hurt or killed. Instead of berating the dibble for having the temerity to not call in the SAS, we should perhaps be lobbying our MPs for legislation to make this sort of thing a specific offence in its own right, without having to wait for unpleasant consequences to trigger a higher offence.