Without getting into a whole English doctorate discussion ...
To me "The QE2" is correct, because there is only one such ship. However, we built 20 Concordes. If there is only one plane flight in the morning I can say "I am taking the early flight". If there are several of them I would say "I am taking an early flight", because it could be one of several. If I say "the early flight" in this context then the person meeting me will have to ask which one.