Code to Show for One post only in head

Write a function, and define it as a callback on the wp_head hook:

<?php
/**
 * print a alternate link on head for a several post
 *
 * @wp-hook wp_head
 * @return void
 */
function wpse_162849_print_alternate_link() {

    //don't do anything expect on singular pages
    if ( ! is_singular() )
        return;

    // check the global post
    if ( empty( $GLOBALS[ 'post' ] )
        return;

    // compare the post-ID
    $your_post_ID = 42;
    if ( $your_post_ID != $GLOBALS[ 'post' ]->ID )
        return;

    echo '<link rel="alternate" hreflang="en-gb" href="http://abc.com"/>';
}
add_action( 'wp_head', 'wpse_162849_print_alternate_link' );

Change the post ID to your needs. You can also implement another logic to choose the right post, for example by a special post meta.