Pitch up authority
It is totally incorrect to say the aircraft may have operated outside the operating envelope. You clearly do not have any understanding of Boeing 777 limitations and your comment infers that the crew may have done something wrong which is certainly not the case.
Unlike some other aircraft the 777 does not have a minimum operating outside air temperature limitation. The only limiting factor is the fuel must not get colder than 3 degrees above fuel freeze temperature of -47C, ie -44 C. In the case of the BA38 the fuel temperature never got below -35 degrees
Unlike the 747 wing the 777 wing is such that cold fuel temperature is rarely a problem even on winter flights over Siberia. So although 747s on the same routing that day had to descend to stay within air temperature and fuel temperature limitations the 777 did not need to. Additionally even at its higher flight level the fuel temperature of MMM would have been warmer than that of 747s which had had to descend to lower flight levels due temperature. The 777 RR engines have since been modified, the limitations on fuel temperature have not.
Welcome back to BA Pete. It's bad enough having idiots in the BA cabin crew training fraternity spreading untrue rumours about you let alone people like Pitch Up Authority casting aspersions on what is supposed to be a thread about welcoming you back.