A family of Microsoft presentation graphics products that offer tools for creating presentations and adding graphic effects like multimedia objects and special effects with text.
You may see this problem if the presentation itself has been set to some language other than the one you want to use. The presentation's language becomes the default, and any pre-existing text will also be set to that other language.
As a test, select some of the text that's giving you unexpected spelling warnings.
Then on the Review tab, click Language | Language Settings
Choose UK English
Does that solve the problem (for that one text box, at least)?