Here is the reply. However you can still book the service on East coast trains website so I guess you will be able to on all others.
London Terminals to Stratford Regional station
Further to your enquiry, I thank you for sending a copy of your ticket so that we can look into the problem that you encountered.
I can confirm that the London Terminals to Stratford Regional ticket you have sent is not valid on your specified route between St Pancras and Stratford Regional. The ticket is valid for use between Liverpool Street station and Stratford Regional station.
Whilst the ticket does state 'any permitted route', the Highspeed route is not a permitted route for this ticket. This is because Southeastern do not receive any of the money for this particular ticket as the route is operated by another Train Operator. The ticket is also not marked with a '+' symbol so it is not valid on most parts of the London Underground network. However I note that there is an easement permitting use of the ticket on the Central line between Liverpool Street and Stratford Regional only.
I am aware that this particular journey has caused us problems in the past, due to an error on our ticketing system which resulted in the ticket displaying as valid for the route on our and National Rail Enquiries' website. This was brought to the attention of our Website and Fares Managers, and I believe this has now been corrected. Whilst the route is still shown on our website if you enter 'London Terminals' (or 'London all stations') and 'Stratford Regional' as the origin and destination stations, with 'Stratford International' as the via station, it appears that the tickets cannot be purchased online and the route has been disabled.
If you wish to travel from St Pancras to Stratford Regional station, you will need to purchase the ticket routed 'London Terminals' to 'Stratford International' with a 'Plus High Speed' routeing. This will also permit travel on the bus service to Stratford Regional, and the ticket is priced £3.55 for an Anytime Day Single with your railcard.
I am sorry for the inconvenience that you have been caused by this, and thank you for bringing your concerns to our attention. I trust this letter clarifies the position and you do not encounter similar problems again.
Yours sincerely
Daniel Westlake
Customer Relations Officer
Southeastern