I have no idea what a Metonym is,
People use them all the time and don't realize it.
The best definition is Lakoff's IMO ""to take one well-understood or easy-to-perceive aspect of something and use it to stand either for the thing as a whole or for some other aspect or part of it"
Examples are things like journalists using "The White House" when referring to the US President or his Administration. "
The White House believes Obamacare is a good thing."
I believe your journalists use Downing Street the same way to refer to the Prime Minister or the active administration.
Also, "Lend me your
ears" in which case ears is substituted for listening attention.
Using the countries name in place of the government itself or the economy. "
The United States believes North Korea is guilty for the Sony Hacks" where the country is substituted for people like the FBI and Obama himself.
Here are some other examples from the Metaphors We Live By book which also points out how a lot of speech people use is metaphorical not literal (especially fans of sports teams - "upward and onward" "we go forward" etc are all metaphors).
OBJECT USED FOR USER
The sax has the flu today.
The buses are on strike.
CONTROLLER FOR CONTROLLED
Napoleon lost at Waterloo.
A Mercedes rear-ended me.
INSTITUTION FOR PEOPLE RESPONSIBLE
Exxon has raised its prices again.
You'll never get
the university to agree to that.
THE PLACE FOR THE INSTITUTION
The White House isn't saying anything.
Wall Street is in a panic.
THE PLACE FOR THE EVENT
Remember
the Alamo.
Watergate changed our politics.