Dave suggests that pilots should be able to abbreviate the callsign when he/she believes that satisfctory communications have been established. Bad idea, the controller knows what else is going on and needs to know what each aircraft is calling itself.
Granted, many pilots will abbreviate the callsign and most of the time it really doesn't matter and the controller was about to do it anyway. But the rules are there for a good reason.
5mb, the AIP doesn't permit pilots to abbreviate a callsign of his own volition. The definitive document appears to be CAP 413 and that says
After satisfactory communication has been established and provided that no confusion is likely to occur, the ground station may abbreviate callsigns. A pilot may only abbreviate the callsign of his aircraft if it has first been abbreviated by the aeronautical station.
Let's stick with the present rules!