Here's another wrinkle, by email overnight they said:

"touch your Oyster card on a yellow reader at the location shown above
as you start a journey."

Which makes a choice of station even more complex (if they mean it). I
can't predict in advance what station I'm going to be using to travel
home *from* (in central London), even though I'm fairly sure which
station I'm heading *to*[1]. It depends what pub we go to after the
meeting etc.

So it has to be St Pancras (or if I was wider awake when answering "what
station am I changing to the tube at") Kings Cross.

[1] Barring disruption of various kinds. My most recent tube journey was
frustrated because the station I wanted to travel from, was closed due
to broken lifts.
--
Roland Perry