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.
Unlike the US way of doing it (which is quite sensible), the UK Mode S 24 bit aircraft addresses assigned by the CAA in no way bear any resemblance to an encoded aircraft registration. The ADS-B broadcasts from the aircraft picked up by an SBS box contain the 24-bit address and either a position, Aircraft ID (i.e. callsign / reg) or a velocity vector. It is the Aircraft ID that the SBS uses to provide identity, not the 24 bit address. The 24 bit address has no identity function apart from allowing a ground radar system to uniquely interrogate or for a receiving system (radar, SBS, Multilat, whatever) to correlate replies for tracking.
While a sizeable ANSP like NATS is highly unlikely to use an SBS box for operational use, give it a couple of years and the system pictures will be augmented with ADS-B extended squitter positions (or in some areas, fill in gaps in radar cover). Its a good job that the avionics manufacturers always get it right and the positions reported by aircraft are always super accurate................
RS