Search one custom post type ONLY, disable “all posts”

I guess you mean overriding the post type of the main search query on the front-end. You could try:

add_action( 'pre_get_posts', function ( WP_Query $q )
{
    if ( 
            ! is_admin() 
         && $q->is_main_query() 
         && $q->is_search()
     )
        $q->set( 'post_type', 'film' );
} );

This way you don’t need a secondary search query or mess directly with the globals.

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