London to Lille would be tough against the Eurostar - the market would be implicity reduced to people living very close to Southend as people in London or Lille would likely go by train.
Albert Picardie is in a rural area (ie low population density). Taking people living in northern France to Nice in summer is usually a good bet. Albert Picardie couldn't sustain a Nice route when IGAvion tried. Tough to sell a London route when Lille station is not that far away
Beauvais is a poor man's version of Paris. Again the Eurostar would severely limit the market to residents of Southend.
Ostend might have potential.