SET's pricing manager apparently doesn't like Advance tickets, and may get angry at the thought of you using his shiny new trains on Advance tickets further afield.Even so, I have used the St Pancras service a number of times with advance tickets (if only South Eastern did advance tickets!) to go further afield in the country and been impressed otherwise.
I think the chances of SET doing Advance tickets is very remote, until there's a change of personnel in certain areas.