The removal of a free-hanging swarm in the open can usually be arranged by a local beekeeper free of charge (although payment of travel expenses or a donation to the association will be appreciated).
Do please bear in mind that beekeepers are not a free pest control service, and in some situations you may still have to call in a specialist honey bee removal company to deal with a well-established colony. Make sure you get a quote before engaging the services of such a company.
The BBKA website has information to help you decide if the bees you can see are in fact honey bees. Many people confuse honey bees with bumblebees, especially the tree bumblebee which often will occupy bird boxes for a few weeks.
If you can confirm that you have a honey bee swarm, then you can use the BBKA “Find a Local Swarm Collector” service