\\flight is accomplished from start to finish on three engines with the wind (and no delays) computed from the aforementioned 4-engine journey times planned at M0.86 cruise. To which level would you have to descend in ISA
conditions to completely exhaust fuel by LHR?\\
G'day to you, enicalyth.
Well, for starters, completing the intended journey (in a four engine aircraft), from start to finish, on three would be irresponsible, in my opinion, IE: the intent of the concerned regulations are to expect the engine failure/shutdown well into the cruise, not from takeoff.
Now, I don't fly the B747-400, but I presently do fly the Lockheed TriStar, and if an engine failed in the cruise, the fuel consumption would be increased by 500kg/hr (same for the B707 in years gone by) and the resultant reduced altitude, provided the aircraft departed at max weight, would be in the vicinity of FL250.
You might well not find the winds at that altitude to your liking, so the journey time would certainly be longer as a result, not counting the reduced TAS in the cruise with one shut down/failed.
Therefore, a landing short of the destination might well be required, to enable an approach/landing with sufficient reserves for a diversion.
Those very long overwater journeys with an engine shut down/failed, with few enroute diversion fields available could well be...not very bright, IMHO.
It is not so much 'will the aeroplane do it', it becomes...'is it wise to do so.'
As always, Murphy's law is there to interfere...when you least expect it.
Just my opinion you understand, from much long haul practical experience in 3/4 engine heavy jet transport ops.