I have heard different accounts but the latest information I have received regarding the June plans is that there will not be barriers on the southbound platform, other than the existing gate. This gate will be closed before 3pm, manned 3pm-8pm, and open 8pm-6am.
TfL are investigating the possibility of barriers at the bottom of the steps (which would also seem like a potential safety problem) but that is not a plan for immediate implementation by Southern Railways.