Couldn't understand this. Surely any area unit, if serving, as a unit, the whole of the FIR/UIR, and whether civil or military, should use the name of the FIR as a callsign prefix? In this case, "London Mil", not the very naff-sounding "Swanwick Mil". The fact that they are not located near London itself is irrelevant. ("Drayton Centre" was eventually changed to "London Centre", quite rightly).
Can any military controllers throw any light on it?