The money is made on Antwerp (and indeed other LCY routes) from business and commercial contacts with Central London, not any eastern suburbs which might be towards Southend. The Antwerp diamond traders who go to and fro with their colleagues in Hatton Garden are not going to be impressed with somewhere another 2 hours east down the A13 (which it is in the evening rush hour traffic). And if you think they are going to squash into a standing-room-only train out of Liverpool Street, you've got another think coming.