Waterloo's congestion problems would be a lot less if they announced platform numbers more promptly. The present practice of leaving it until the last minute results in congestion on the concourse as people wait for their train to be announced, and a mad scrum at the barriers when it is finally called, (not to mention the conflicts with people arriving off the train) and because the train is due to depart so soon people naturally jump on the nearest carriage, meaning that people getting to the platform a little later can't get on at the rear and haven't got time to get further down the train, thus getting left behind and having to wait for another train, further adding to the congestion on the concourse.
It is simply not credible that the people responsible for the departure screens do not know which platform a train is to go from until it arrives. (And if a late change is necessary, it would be much easier to manage if the concourse is not clogged up with people).