V1GO
What part of London do you need to visit?
Southend is cheaper than Biggin, and is more convenient for City of London, Docklands and east of London.
Possibly the train fare may be slightly higher, but the frequent and reliable service from Rochford to Liverpool Street station, right in the heart of the city, usually takes 45 minutes.
Taxi from terminal at Southend to Rochford only 5 minutes.