Phileas,
Your logic looks reasonable, but why would an operator of "CRJ, AvroRJ, Fokker/Embraer regional jets" risk SEN as its London terminus when LCY has a much more affluent, closer and larger catchment?
I realize that it is slot-restricted and expensive, but the expense would be offset by higher fares, and slots are presumably available outside peak times (and what would peak times be at Southend?).