Moll...I have been following this thread purely as a source of information on the accident in the Channel.
However, FBW made a very pertinent point, then appeared to get set upon for stating, which I thought, was the blindingly obvious.
If you are speaking with someone on box 1, be it London FIS, ScottIsh FIS, or Heathrow approach, then that would be to whom I would make a Mayday call, in the unfortunate event I had to make one. I do not fly with box 2 on 121.5, maybe some do, but I generally have a pertinent comms frequency on box 2. The set up on box 1 would be who I am speaking to, the standby is who I may speak with next, and the third is in prepartion for either ATIS at my destination, or the next in line.
But each to his own, and everyone fly's their aeroplane, I assume, in the manner they feel safest.