They did indeed divert into Manchester on that occasion. It was discussed to death at the time, and was due to being unsure as to whether all of the fuel in the failed engine’s main tank was available. (Nothing to do with a higher fuel burn than they anticipated). They decided to play it safe, even though it was actually all usable and there was plenty of fuel to continue on to LHR. There really is no reason a 747 cannot continue on three engines as long as all of the continuation requirements are met, however the publicity created around that particular event has resulted in public perception guiding the likely subsequent actions, rather than flight continuation safety. The downgrading of the Mayday in this case, will have been due to the fact that the flight continuation situation was by that stage fully assessed, and there was no longer any particular flight safety urgency to be on the ground - just public perception.