It is even more complicated than suggestions so far! The UK radars (as in the ground based things that spin around) are mostly now upgraded and can receive Mode S, however the equipment in London Area Centre can only display certain elements of the Mode S, and callsign is not one of them!! In the enroute sectors the system can even cross check the level entered into the ATC system against that selected in the cockpit using the Mode S data, and will alert the controller if there is any mismatch. But still we can only see Mode A to identify a track, or just a primary return otherwise.