Wow, very surprised. Normally gets a 124 error.
West Ham station is on the c2c line though, which is direct to Southend Central and Southend East.
Perhaps if you have taken the Jubilee Line to Stratford and gone to Southend Victoria, you might have got that 124 error. (Personally have never tried. Ladyfriend trout will not allow the Stratford detour from West Ham to use a Class 321
and trust me, I have tried
)
The ticket machines at West Ham will sell you a ticket to all national rail stations but you will get the LU pink ticket stock. You'll also find the ticket machines there will issue railcard discounts. Good luck getting the Gold Card First Class Upgrade though