To clarify a couple of the points mentioned above:
The name that an airline gives to an airport in its commercial marketing is very much its own choice, so the use of London Southend by easyJet and others is simply making use of a marketing name. Similarly a number of airports add the prefix London (London Oxford I think is the most recent) for marketing purposes. The difference now for SEN is that throughout the world SEN/EGMC will appear in airline documentation and data sources as London Southend giving it very much greater profile in such areas as route and flight planning. One example would be client requesting an airline or broker for a charter to 'London' whereby SEN will now appear as one of the choice of six London airports on various databases from which the airline/broker will make their choice of destination.
As far as the 738 is concerned it is very unlikely that it could operate with sufficient payload from SEN to make it economically attractive to any airline. It's quite possible that SEN see this as an advantage as it keeps it out of the clutches of a certain large European airline and protects its customer airlines from that carrier's predatory tactics.