As well as the above, they ring an audible alarm in the D&D cell in West Drayton.
Also, some systems will retain the code/callsign conversion with 7700/7600 etc, some won't. That's why I believe it is unnecessary to ask an aircraft with a 'Medical Emergency' who has flown the last 300nm past 4 suitable airfields to squawk 7700 on his way into Heathrow (unless the medical emergency is one of the flight crew)! But the 'Do it by Numbers' members of my profession do not make the distinction or give it any thought.