Googled and it came back with Rochford Gov document which states:
– Potential Catchment Area • Southend has a significant population living within 30 and 60 minutes drive of the airport. • A developed Southend could potentially draw on a large local market to form the core of its traffic base. • Southend Airport’s immediate 30 minute drive time catchment (orange area on map) contains 0.4 million people, and includes Southend, Basildon, Rochford and Castlepoint. • The 60 minute drive time catchment (yellow area on map) contains 2 million people, and includes most of Essex, Thurrock, North East Kent and the London Boroughs of Bromley, Bexley, Havering, Barking and Dagenham, Newham, Redbridge and Waltham Forest.