Don't think that not listing Southend under London will cost it any bookings as UK holiday companies will be selling holidays to the British who will know that Southend is by the sea.
It will be interesting to see if there is a price premium for flights from Southend. Companies like Thomson can load several hundreds of pounds per family for flights that don’t depart from Gatwick.
Might well explain why Gatwick is number one for IT when one of my neighbours who lives no more than a mile from Luton picked a family holiday from Gatwick due to the amount of money he saved despite simular flight times.