Send Clowns
The selection of 7700 will cause flashing and label bright-up on all Area control displays at Swanwick and elsewhere. It will inhibit the ability of the system to display genuine emergencies, because it can saturate.....especially on sunny days when the hangar-queens and "company" aircraft get airborne. If the offender happens to be in a busy portion of airspace.....even though all the other traffic is way above.....the 2-dimensional nature of the displayed information means that labels of airways traffic may be blotted out by the spurious 7700. Remember that many aircraft that use 7000 may not necessarily be in communication with anyone; additionally I should add that transponder confidence or validity checks are available via Londion Flight Information if there is no suitable SSR-equipped unit in your area.