I understand that the engine failed on takeoff at LAX at 100' altitude and the crew/company elected to continue to LHR. Control Tower saw sparks coming from the engine.
While a 74 is ok on 3 engines, it's certainly going to burn A LOT more fuel at lower altitude and higher drag.
I wouldn't go on an ocean crossing with a failed engine due to unknown reasons, unknown damage, unbalanced windmilling, etc. Could the reason the company elected to continue with a disabled aircraft have anything to do with the new passenger compensation regulations?
You would also have to consider the possibility of the other engine on the same side failing. If that happened you'd be in a lot of hurt both fuel wise and landing controllability wise. Also, I'm not sure on the -400, but on the -200 depending on which side the engines are out on, the gear will go down but won't come up, hence probably no go-around capability.