Callsigns are set by the crew as Heathrow Director points out, however I believe these are broadcast from the aircraft not looked up by an ATC computer, hense the silly ones seen!!
Not correct. Callsigns seen on the label display on the radar (which, by inferrence is what your original question asks about) are derived from the ORCAM code that is paired up with a flightplan callsign.
The mode S data (within NATS equipped units) which is always shown, is the SFL.
Mode S data which is available via selection globally (i.e you toggle it on and off and it shows on all targets on your screen) is IAS/HDG/GS.
Mode S data which is available via individual aircraft selection (comes up in a separate box for a specific target) is c/s/HDG/IAS/GSD/ROD/ROC/ etc