[Neil Jones]

It's used by nuclear subs too, even to this day, to determine if the UK exists still...

Yeah I think that might be a bit stretching the truth. I think one of the check is that Radio 4 is on air, not necessarily on long wave as traditional radio waves don't travel well in salt sea water. It's more likely to be a rebroadcast from other similar technologies.

But Radio 4 is not the only check in that regard, though it is probably the most well known. Its not a case of "OMG the Today programme isn't on, let's see if we can nuke somebody".