This incident is not really about the letter of the law it's about the intent or spirit in which it should be applied.
Clearly a modern 4 engine aircraft is designed to be operated for extended periods of time on 3 motors when sound professionalism and good airmanship have been applied, and of course when commercial aspects have been considered.
However, I don't think it was ever intended that a aircraft could have an engine shut down before the flaps were up and then its flight continued for thousands of miles including an ocean crossing, regardless of how many alternates were on the route.
Just because an aircraft can do such a trip does not mean it should! If BA was to be honest with its self now I don't think they would make the same decision again should something similar occur in the future.