The country code is "GB".... but the new checkout doesn't store all of the form fields individually any more.
To get the necessary field stored into the session, update your confirm_cart.php page and add this code below the cart's ->getContents() call:
<?php
if (isset($_POST['Country'])) {
$_SESSION['eCartCheckoutForm_shipping_country'] = $_POST['Country'];
}
?>
That way the session variable you are referring to will be set and the shipping should be calculated properly.