It’s BBC 4.
Because submarines normally function in complete radio silence, their ability to communicate with the outside world is impeded to a significant degree. During the entirety of their mission, they are required to remain invisible and silent; yet, it is imperative that they learn whether or not their country is still in existence.
At least in the Royal Navy, one method that is utilised is to check to see if BBC 4 is still being broadcast. This is done in order to determine whether or not the channel is still active. If for some inexplicable reason it does not, the current administration of the United Kingdom will probably certainly be overthrown.