GPS and Distress calls
Although distress calls are (of course) transmitted in a vastly different part of the EM spectrum from GPS, disabling GPS disables a crucial part of the Global distress system (GMDSS). A VHF technology called DSC provides the capability to broadcast digital message containing latitude and longitude; these are used heavily in distress messages that are provided on pushing a Big Red Button. Of course, the latitude and longitude are normally provided by a GPS system; other types of electronic position finding are rarely used.
Automated distress beacons also provide positions from GPS; again, although the communications technology is not compromised, the GPS is. In this case, there is no alternative technology feasible.
However, the other side of the coin is that I, an amateur yachtsman who happens not to have gone to sea for a couple of months, knew all about these tests. I really don't understand why the fishermen didn't know about them - they are routinely notified through a wide variety of channels. What do the fishermen want? Personal visits from HRH?