Southend is actually more convenient than Gatwick, if you can fly into there, if you are going to East Kent, but not too many people have realised that yet.
And that would be because according to my research, LGW is quicker to get to from East Kent by a good 15 / 20 minutes and SEN takes just as long as STN. Via road:
Canterbury - LGW 1 hr 12 mins,
SEN 1 hr 24 mins, STN 1 hr 25 mins
Folkestone - LGW 1hr 10 mins,
SEN 1 hr 30 mins, STN 1 hr 31 mins
Maidstone - LGW 44 mins,
SEN 1 hr 3 mins, STN 1 hr 4 mins
And even from East London, SEN still doesn't quite win (and gets worse further into London when compared to STN):
Canary Wharf - STN 42 mins,
SEN 53 mins, LGW 57 mins
Dartford - LGW 42 mins, SEN 45 mins, STN 46 mins
Romford -STN 37 mins,
SEN 40 mins, LGW 58 mins
And Essex:
Basildon -
SEN 24 mins, STN 41 mins
Braintree - STN 37 mins,
SEN 1hr 6 mins
Chelmsford -
SEN 31 mins, STN 33 mins
Colchester - STN 45 mins,
SEN 52 mins
Harlow - STN 20 mins,
SEN 53 mins
Source: Google Maps
So it is literally the from outside the M25 eastwards and no further north than Chelmsford where SEN is actually the closest option. So I re-iterate my point that SEN covers a much more local catchment area than the other airports.