The Services Add-In for ArcGIS for Desktop lets you see the premium Esri UK Online Services that you have access to and provides an easy way to add them to your map. If you have a full license of LocatorHub, this Add-In can also be used to connect to your gazetteer services. For more information about the premium services see here.
The Add-In adds a toolbar into ArcMap called Services and is compatible with ArcGIS for Desktop 10.2 and above (see the release notes in the download for the version required for different releases for ArcGIS).
The Services Add-In is an open source project produced by Esri UK and is provided to customers free of charge.
Note: The Services Add-In is is now in mature support and is no longer appropriate to access the Esri UK free online services (maps and locators) which can be accessed directly using out of the box tools. See here to find out more about how to use them.