Guido, I believe terrorists can be aware of transponders - the 9/11 guys turned them off. Plus, its not exactly difficult to get the information. All the ATC books I borrowed from the library have the SSR squawks in.
So the question now seems: if the pilot was to return to LHR, by VFR or IFR, how would he know which path to take back without causing too many airproxes, and how would the controller know what was going on? I get the impression that the London TMA and LHR approach are kinda busy!