Here we go again…..
Not sure where we are nowadays with flight continuation but certainly once upon a time under some jurisdictions continued flight on a four engined type reduced to 3 engines was acceptable as long as the rest of the route allowed further degradation to 2 engines, so you had to look hard at en-route MSAs vs 2 engined performance, and suitable and available en-route alternates
It was certainly allowable to continue on 3 engines with passengers on-board and I even ended doing it myself on one memorable sector.
As Meikleour says the main reason the BA flight made it into the headlines was fuel handling not an actual shortage of total fuel on board, and I think many of those at the company learnt something from that.