"a union flag" or "the Union flag"

[Murom]

The prime minister signed the letter shortly after 4.30pm in the cabinet room in Downing Street, next to a union flag and beneath a portrait of Britain's first prime minister, Sir Robert Walpole. © theguardian.com

Dictionary.cambridge.org says Union falg = Union Jack and gives an example: The Chinese anthem was played after the Union Jack was lowered in Hong Kong for the last time.

So is it right to write "a union flag" or it should be (always) written as "the Union flag"?
Thank you.

 

[Stoggler]

I would say that you write it as Union flag. If you were to not use the capital letters it could be a flag of any union (i.e. one not necessarily specific to the UK).

The Guardian's style might be to use lower case but I think that's too ambiguous.

 

[Keith Bradford]

But as far as the article a/the is concerned, it's the same rule as anywhere else: "The Union Flag is often referred to as the Union Jack" compared with "I saw a Union Flag pinned to his bedroom wall, so I assumed he was a fascist".