The flight number and callsign are, in my experience with BA totally unrelated. The shuttle callsigns are for domestic flights with Speedbird being used for international. I think cityflyer use a different callsign also. As far as I'm aware, the alpha numerical callsign is purely to avoid similar callsigns being in the same sector at the same time, although the same callsign allocation plan is usually repeated day after day (if it ain't broke, don't fix it!). When it all gets too complicated, it's not unusual for operators to use a second voice callsign as happened recently with BMI and the introduction of the callsign Kittywake.
Anyway, enough geekyness for tonight!