Advances from Southend Airport to Sandwell & Dudley are set and controlled by Virgin Trains West Coast (VTWC). Their availability is limited by both legs of the journey - Southend Airport to London, and London to Sandwell & Dudley.
VTWC have not yet released Advances for Sundays beyond 7 October. This will be for a variety of reasons, including that they do not yet have confirmation from Network Rail as to what engineering works and closures may be occurring, and hence what trains will be running and at what times. This is because Network Rail typically do a lot more maintenance and engineering works during the weekend, when there are fewer people using the trains.
However, Chiltern have already released Advance tickets quite far in advance (as they have a much smaller route and far fewer services, so Network Rail presumably are already able to confirm works and timings), so 'Chiltern+Connections' Advances are already available for your desired dates.
They are available for exceptionally reasonable prices too - it costs just £11.00 (without a Railcard) if you obtain an Advance taking you from Southend Airport to Liverpool Street, Tube across London and then Chiltern from Marylebone to Birmingham Moor Street, West Midlands Trains from there to Smethwick Galton Bridge, and then again one stop to Sandwell & Dudley. This is available for departures every half an hour throughout most of that Sunday.
You won't see that kind of ticket by default, however adding a 'via' point of High Wycombe ought to get it to show in most good websites like
www.trainsplit.com.
