But if you're well out to sea, you probably wouldn't get a call picked up by either London info or 121.5.
Surely here the point of 121.5 is that there is likely to be some high level airline monitoring the frequency and could relay the message.
As above though it's unlikely the pilot made a mayday at all in this case, as he was probably busy trying to deal with whatever situation developed, but in other cases a mayday may be useful, and the choice of frequency could mean life or death.