Thompson-Reuters was once formed out of the Thompson Corporation and the Reuters Group plc news agency in 2008; at least that's what Wikipedia offers as a base information about it. Thompson had his headquarters in New York, Reuters in London, so I guess it must be Reuters when there is only one office of a news agency in London. (Generelly there have never been many great news agencies in the world, so the choice to is very limited.)
And always if you hear anything about the Syria war, the way of the source goes back to Reuters in western hemisphere.

The thing is even getting more funny, if you type some of the names who provide news to the world into the search engine.
Always you find out, the institution belongs to various other interest groups of that industrial sector in shares.
At least for where news come from in the German land I stumbled upon this.

So that makes no wonder why you sometimes have the same information or even the same pictures going round in several countries. If one owns the other - it even makes sense in the case of copyright issues and cost minimization.

If you want to name it that way - well, do you know the human saying of "the 4th power in government"? Guess they didn't think that up just for plain amusement.