I think you can be sure that Adria Airways would not be paying more to use Southend than say Luton or Stansted. In fact they are probably paying less. That might change after any discount period runs out or if they can get a slot at Gatwick which is their preferred airport and where their other London flights operate to.
Rail - Over the Easter Weekend (Friday 03/04 - Monday 06/04) there will be planned rail maintenance work on the Abellio Greater Anglia Line which will affect the rail line between the Airport and Central London. There will be buses in for trains and a revised service running.
Road - On Monday 06/04 there will be a charity motorbike ride which will be using the A127 to go from London to Southend.
We would advise all passengers to allow extra travel time.
Sometimes events conspire to make life difficult for passengers.