WordPress Mysql query and Duplicate

There’s no need for custom SQL here, use the power of meta queries:

$posts = get_posts(
    array(
        'posts_per_page' => -1,
        'meta_query'     => array(
            array(
                'key'   => 'Country',
                'value' => 'UK',
            ),
        ),
    )
);

foreach ( $posts as $post )
    $cities[] = get_post_meta( $post->ID, 'city', true );

foreach ( array_unique( $cities ) as $city )
    echo $city . '<br />';