I'm not a prefessional pilot, and not quite a PPL yet, but in my ignorance, I'll throw in my two pennies worth.
As a passenger, on a 747, given the option of either turning back due to engine failure, or continuing on three healthy engines, I'd happily continue. I would not want all the grief to be had in turning back...a slower, lower flight to my original destination would be the better of two evils.
As for fuel, I'm guessing that in this case the pilot could not know for sure whether the prevailing conditions would allow him to reach Heathrow. But he must have been sure that, given any conditions (including possible 2nd failure), he had enough fuel to get as far as BFS / PIK / MAN / SNN etc. So worst case, the a/c doesn't reach LHR, but at least gets the right side of the pond.
And given that PAN and MAYDAY are the only 'emergency' options available, what else could the crew do but to do what they did? Maybe they could have diverted a little earlier (BFS maybe), but would that have avoided the MAYDAY call, or would the fuel level have been low enough even then to trigger the 'emergency'?
Now, I say all of this as someone with some aviation knowledge...an ordinary passenger, just wanting to get home, may not have been quite so relaxed!