"Neil Williams" wrote

About an hour later, an announcement was made that platforms 8-11
would be closed and that people should return to the concourse. This
was said to be due to dangerous overcrowding, which was not evident
from where I was sitting.

A further 15 minutes later, a member of staff came through the train
chucking everyone off (fairly rudely), and the unit was locked OOU.
It didn't, however, go anywhere.

I can think of some good reasons why a train which has been indefinitely
delayed at its starting point should be cleared and locked out of use.
Unfortunately the reason you were given was not one of them - so if they had
a good reason they should have told you the truth and acted firmly but
politely to clear the train.

The train crew may have been running out of hours - unless another crew had
been assigned to take over they would have to shut down and secure the
train.

Control may have wished to recover the service by running the first train
able to leave fast to, say, Milton Keynes - so it wouldn't be overwhelmed by
hordes for all stations along the line, who could have been directed to a
second train a few minutes later. They wouldn't want there to be passengers
on the train for intermediate stations, who may miss announcements, or who
may wish to try to leave the platform as a full trainload are trying to join
the train.

Peter