Using 'Adjacent Departure Airports' - VATSIM-UK/UK-Sector-File GitHub Wiki
As London or Scottish Control sectors*, there is the option to choose whether departures from adjacent airports are displayed, by means of a special runway 00 at '000A Show adjacent departure airports' in the Runway Selector dialogue.
Option 1: with a selected runway 00 on the right hand side (under DEP or ARR, or both, doesn't matter): the departure list will show departures from all airports within the sector, or immediately adjacent to it.
Option 2: with no runway 00 selected: the departure list will show departures only from airports for which you have top-down responsibility for TWR services. That is, only when nobody else is covering it.
*(including as an observer, using Alternative Sector Ownership)
Thanks to Jamie Fox for originally implementing this option, and some of the instructions above.