As far as I am aware Dublin might be the only other destination served from all five London Area airports. Are there any others?]
Aswell as:
Glasgow
LHR (BA,BD)
LGW (BA, U2)
LCY (BA)
LTN (U2)
STN (U2)
Barcelona
LHR (IB, BA)
LGW (BA, U2)
LCY (BA)
LTN (U2)
STN (U2)
I'm sure there are more.