In the second example, the final two words don't even represent the country "the United Kingdom". The film is about "a united kingdom". The only reason all three words are capitalised is that it's a title and that's the format for a title.
The same goes for "A United Germany" - it's the title of an article and is therefore capitalised correctly. It was written in 1990, not long after the fall of the Berlin Wall so it refers to the prospect of "a united Germany" - ie a single country, no longer divided into East Germany and West Germany. If you like, think of it as "a version of Germany in which it is no longer divided into two separate nations".
(I've corrected your link so it takes us to the preview of the article.)