This afternoon I took a Thameslink train to Sutton which
arrived at St. Pancras International at the "wrong" platform.
This was clearly intentional as the train announcer warned
that the doors would open on the right-hand side. (There
was a train waiting at the other platform which seemed to be
heading for Bedford.) On our platform none of the destination
indicators listed our train or Sutton.

Is two-way running common at St. Pancras International? If it is,
why do the destination indicators not display South London
destinations?