FWIW, it's not as easy for Southend. If travelling from London Terminals you can't get a ticket to Southend Stns, but you can get a ticket to Southend Stns from many stations outside London.It's clearly an anomaly. When the new station came into existence I think someone forgot about SAA. I think that SAA and SAC should both be part of a new St Albans Stations group. It exists for Southend, Pontefract, Bradford, Hertford and many others... why on earth not for St Albans?
Some stations within the London Zones (eg. Kensington Olympia) don't have a fare to Southend at all whereas others (eg. only have fares to Southend Stns). All the boundary zone fares are to Southend Stns only.
I think, after having looked at City Thameslink, the rule is that any ticket from Zone 1 is only printed to the individual stations, and any ticket from outside Zone 1 is printed to Southend Stns, the exceptions being Stratford, mentioned above, or West Ham which can issue tickets to the individual stations or Southend Stns. Interestingly enough, West Ham - Southend Stns CDR is £13.70 (set by NX), whereas to Southend Central it's £12.00 (set by c2c).
Interesting routes like Stratford to Southend Vic is routed "+Romford" whereas I cannot see any use for the tube. If you put in Southend Central it automatically changes it to Southend Stns route "Any Permitted," yet from what I can see it wouldn't be possible to get to Central, unless that ticket is valid via Liverpool Street & on LU to Tower Hill, or on the Jubilee to West Ham.