@bbg
Unless RR has improved its powerplants exponentially the references to 45000,(FL450) is unavailable because at the weights that the 777 was at it would only get to FL390 near the end of its scheduled flight.
I believe that BA38, the 777 that crashed at LHR, had spent a lot of its flight at 40,000ft? That was thought to have contributed to the icing problem.