Much like booking F on AA for a transcon, F on LHR-DME seems like a pure ego flight. I did LHR-DME when it was still the old CE service but with a LH 767 and thought that the seat itself was more than adequate for a flight of this length.
Does the use of 744s have anything to do with cargo contracts on the route (since nobody else uses an aircraft even close to this size on this route)?