Here’s a way to get the top page url:
$top_page_url = get_permalink( array_slice( get_ancestors( get_the_ID(), 'page' ) , -1 ) );
where get_ancestors() returns an array containing all the parents (ID) of the given page. You can read more about it in the Codex here.
Here are various ways to get the last array item, but note that end() doesn’t expects a function as an input – more about it in the PHP docs here.