Rated 5 out of 5 stars
Good to see British English back on the add-on menu, no more seeing red over the wrong color without this customisation.
The install instructions refer to a "multi-line text box". You do indeed need to find an on-page web form in which more than one text line can be entered in order to get a "Languages" menu item on the right-click context menu. Then you can set "English (United Kingdom)" as the spell-check dictionary. Once configured you spell check single and multi-line input fields/boxes in the normal way.
Here's a form with a multi-line text field to use to get the configuration done: http://www.freedback.com/examples.php