Firstly, public $helpers = array('Html');
will not load the helper in your controller, it will load it in the view file rendered by your controller. You will be able to use the $this->Html reference in the view rendered, not any controller action. This is regardless of whether you try to load the Helper in a plugin controller or a normal one.
Check the package tree in the Cake Docs and read more on the differences between a Component and a Helper. You can use a Component in a Controller and a Helper in a View. You should not have the need to use the HtmlHelper (which assists with creating HTML form elements) in the controller. That would break the MVC paradigm.