There is absolutely no difference between Ba, cx, and sq's definitions of a flexible ticket. As mentioned by Anonba, this is about allowing changes and refund without a fee.

Neither airline waives fare difference if there is one, and service charge is what it costs to do something over the phone that you could do (service charge free) over the Internet.
No, it is a flexible ticket to begin with. My interpretation of flexible ticket means I can change my date / time of flight. At least that is how it was with SQ and also CX which I fly quite often. BA's interpretation is that I am allowed to change the flight, but in exchange they see it deem fit to charge an exorbitant amount of "fare difference". That's as good as telling me the passenger I can't change. Perhaps I should have read the fine print. Guess I will give this airline a pass from now on.