As a footnote, the double PAN-PAN is a later introduction that clearly eluded me, as i stopped active controlling in 1983. Certainly in 1963 (Civ ATC) and 1965 (Mil ATC) I was taught PAN x 3, and used PAN x 3 whilst doing my PPL in 1963 in anger!

The US approach is indeed different. In the 1970s I used to lecture at USAF instrument Schools, for new arrivals in UK Airpace, and had to emphasise the need for a simple MAYDAY/PAN instead of “I have an emergency” for best response from UK ATC. I also emphasised that use of 243.0/121.5 (the emergency frequencies) was free at the point of use and would provide a similar best response, thanks to auto-triangulation at the RAF Distress & Diversion Cell at LATCC(Mil), RAF West Drayton ... now at Swanwick, of course.

https://nats.aero/blog/2014/08/distress-diversion/
http://flyontrack.co.uk/wp-content/u...2013/08/DD.pdf