ok digging deeper in WA_eCart_USPS_PHP.php on line 87 you have:
$USPS_XML .= "<Country>" . WA_GetCountryName($USPS_Parameters["ShiptoCountry"]) . "</Country>";
and the function WA_GetCountryName() which is in WA_eCart_Definition_PHP.php converts like so:
case "GB": return "Great Britain"; break;
so I can add my own cases to that switch statement as here in the UK the address would be UK and not GB i.e.
54 Some Place