But how they can get access to all this data flows? They are physically connected to different undersea communication cables. On closer inspection you can see that U.S. and GB are quite important intersection points in the global undersea communication system.
To get an overview about these worldwide infrastructure, Greg Mahlknecht shows a nice webmap on his site: http://www.cablemap.info/.
In addition to the map you can download the raw data and get a list of cables.