Suppress 'Unsupported locale specified to parseStreetAddress: en_GB' message
If you switch off the checkbox for street parsing (as petednz suggests) the error will disappear. But if you really need the street parsing that will not help you. In that case you would need to add the en_GB to the array in CRM_Core_BAO_Address in an extension, like so:

  static function parseStreetAddress($streetAddress, $locale = NULL) {
$config = CRM_Core_Config::singleton();
/* locales supported include:
 *  en_US - http://pe.usps.com/cpim/ftp/pubs/pub28/pub28.pdf
 *  en_CA - http://www.canadapost.ca/tools/pg/manual/PGaddress-e.asp
 *  fr_CA - http://www.canadapost.ca/tools/pg/manual/PGaddress-f.asp
 *          NB: common use of comma after street number also supported
 *  default is en_US
 */
$supportedLocalesForParsing = array('en_US', 'en_CA', 'fr_CA', 'en_GB');

if (!$locale) {
  $locale = $config-&gt;lcMessages;
}


and this is a bit awkward :-)