I know there is What3Words - although there have been cases of people mishearing words so they can't find the location.
I have 2 things on my phone - one is OS Locate - which gives an OS Grid reference.
e.g. Scafell Pike is NE215072
I also have an app that has a compass on it as well as the latitude and longitude indicator.
I think both these work without a mobile signal...
Never had to use it "in anger" but I hope they could be of use if I had to give a map reference to emergency services.
Just thinking about this in relation to previous news cases