I can see that my og:locale is en_us
But many of my sites are in the UK and facebook appears to allow en_GB
Can this be part of the SEO Booster plugin in future to save me editing PHP files?
The locale is being taken from WordPress. This can be configured within the wp-config.php file like:
define ('WPLANG', 'en_GB');
You can find more about that on WordPress: http://codex.wordpress.org/Installing_WordPress_in_Your_Language
I hope that helps.