IF you are in a 4 engine plane and lose one engine, you can fly anywhere you like for as long as you like...
No, you can't
You have to meet the same performance requirements that any other transport aircraft meets - basically that the failure of a critical engine anywhere along the route (or a depressurisation) will not result in being unable to maintain terrain clearance, or not have sufficient fuel to continue to an adequate airport. Obviously the most limiting case (the take-off) no longer applies, as the aircraft is already airborne. The BA aircraft met those requirements.