If an airline operated from DTV to Southend, then anyone travelling from Darlington or Middlesbrough to London would almost certainly just take the train instead.
The train from either of these stations to London takes no more than 3h01 minutes and runs every hour - a flight from DTV to Southend cannot possibly compete on frequency, door-to-door-time, cost or on general comfort (i/e. relatively uninterrupted journey with space for work).
If a route from DTV to Southend were to exist, it would have to be for people travelling specifically to/from south Essex rather than central London. Perhaps there are enough to fill an Eastern J41, but the passengers probably wouldn't be willing to pay Eastern's high fares compared to the train.