Anotherthing is somewhat misleading here - NATS has upgraded some radar sites (CLH, CLX, DEB, Glasgow, Belfast, Allanshill, Lowther Hill, and Sumburgh) which carry a unique code so that only that radar can process the reply from the a/c. These upgraded radars also interrogate on Mode A & C, and the Mode S information is fed into the FDP system via a different route, such that a controller without access to a Mode S enabled radar will still see the information displayed.
This explains why units with Mode S enabled radar displays (MACC, TC and NPC) that have enhanced capability can identify the registration of an a/c squawking 7000, even though the a/c is not displaying a squawk allocated by NAS. This is the feature which enables spotters with SBS to see a/c registrations, as it is part of the encrypted 24 - bit address of the a/c. Very useful for identifying infringers too!