As soon as you nail a box to a Woodland Trust tree you may well break a law.
When you see a swarm on private land, by all means ask permission to take it, but the window of opportunity is narrow before it is no longer a swarm.
In property law, honey bees fall into two categories:
animus revertendi, a hived or nested bee which intends to return to that site and remains the property of the beekeeper or property owner.
Ferae naturae is a wild creature, does not intend to return and may be taken and become the property of anyone (provided, I assume, that permission of the property owner is granted).
This googlable status was established by the Romans and applies also to many other human actions; reference will be found in the Frimston & Smith booklet
Beekeeping and the Law: Swarms and Neighbours.
This seems a strange way to establish ownership, because after 2 days it will no longer be
ferae naturae (a swarm) but
animus revertendi, an established colony owned by the person on whose property it has settled. Fian's description suggests that whether or not the beekeeper pursues the colony in the generous 2 to 20 day window, it remains the beekeeper's property regardless. All very odd.