This is a pretty disgusting way for Southern to treat their customers to be honest. Not impressed with this news at all.
However, if enough people are against this, I think the most effective challenge is to kick up a stink about the lack of disabled and pram/wheelchair/heavy-luggage access - at all hours, but especially when the side gate is closed.
I'm thinking that tfl will have to do something about this when they take over; so why not pressurise them to start the work now?
Perhaps a locked gate with step-free access at the post-office end of the platform (which can be left open afterhours like the gates on Platform 1 and opened as required by staff during the day)?