A regional airport an hour away from central London via the train? Just waiting for 'London Southampton Airport' now!
Although on a more serious note Southend needs its London tag to attract passengers as few people outside the region know about 'Southend' which is really a small seaside town in Essex whereas in comparison Southampton is large city known for being one of the main Ports to/from the UK and thus is better known, hence why a London tag would not have as much of an impact and would still look silly!