When I force en-GB from user.comfig the date picker in /WebInvoice/Edit.aspx shows correctly as dd/mm/yyyy not as US format mm/dd/yyyy
<add key="UseCultureOverride" value="true" />
<add key="site1culture" value="en-GB" />
<add key="site1uiculture" value="en-GB" />
The bold one is the one that affects date formatting, the other one just controls which resource file is used for labels.
Unfortunately we just don't have the data for UK states in the database. You could add it manually from Administration > Core Data > Geographic Zone Information. Once you add it there then it will populate the dropdown.
Hope that helps,