Within your account, you can set default region settings, which will be applied to all new titles when they are created. This will dictate on which NetGalley site your books appear (NetGalley.com or NetGalley.co.uk or both) and which regions can request titles.
To do so, click your Company Name at the top of the account (next to the silhouette icon). In the side-bar at the left, look for "Defaults for New Titles" and click the orange arrow next to "Region."
You'll now see the following options: