You can use the depth parameter like this:
wp_nav_menu( array(
'menu' => 'header-uk',
'menu_id' => 'nav',
'menu_class' => 'sitemap',
'fallback_cb' => false,
'depth' => 1,
) );
Or if what you want to do is grab all items to create your own loop/output you can do that as well. Example:
if ( ( $locations = get_nav_menu_locations() ) && isset( $locations[ 'header-uk' ] ) ) {
$menu = wp_get_nav_menu_object( $locations[ 'header-uk' ] );
if ( ! empty( $menu ) ) {
$menu_items = wp_get_nav_menu_items( $menu->term_id );
if ( $menu_items ) {
foreach ( $menu_items as $key => $menu_item ) {
if ( $menu_item->menu_item_parent == 0 ) {
echo '<a href="' . esc_url( $menu_item->url ) . '">' . $menu_item->title . '</a>';
}
}
}
}
}
The key is in the menu_item_parent check which makes sure an item isn’t a child item before echoing it.