A few months ago I phoned Essex Police (via 999) to report a dangerous obstruction on a major road. Because it was part of the network of roundabouts and slip roads near Stansted Airport, I didn't know the exact name of that road- I could only say that it was the slip-road onto the A120. The 999 operator advised me to use W3W to give the exact location.

Now, this is only one anecdote, and maybe I just happened to get the one operator who had swallowed the what3words PR line- but at least some British emergency services are telling callers to use it.