Even though the terms "TV Show" and "TV Series" are mostly interchangeable in many parts of the world (I don't have a better name idea), antp has spelled out the intent for these two categories so many times in comments and in the forum that it shouldn't still be an issue.
(I would say an explanation of TV Show/Series should probably be added to the "Add a Movie" page)

For the Patridge special, TV Movie seems good enough to me.

It won't always be a perfect fit, but I don't think there are enough of these types of TV or Video releases to warrant a new category.

My two cents