ConfigSettings/en-GB-RegionalSettings.xml

<gs:GlobalizationServices xmlns:gs="urn:longhornGlobalizationUnattend">
 
    <!--User List-->
    <gs:UserList>
        <gs:User UserID="Current" CopySettingsToSystemAcct="true" />
    </gs:UserList>
 
    <!--User Locale-->
    <gs:UserLocale>
        <gs:Locale Name="en-GB" SetAsCurrent="true"/>
    </gs:UserLocale>
     
    <!--input preferences-->
    <gs:InputPreferences>
    <!--en-GB-->
        <gs:InputLanguageID Action="add" ID="0809:00000809" Default="true"/>
    <!--en-US-->
        <gs:InputLanguageID Action="remove" ID="0409:00000409"/>
    </gs:InputPreferences>
 
</gs:GlobalizationServices>