I have written to Southern and this is their response (see below). I am going to find out who we can write to and complain as it seems as this idea is ill thought out and will cause a lot of inconveniance. It has also been forced through rather quick whilst for many years the station has been left to rot..
The planned gate opening for Forest Hill is 7th July, we will not be accepting pay as you go until we sign up next May. The gate on platform 2 will be manned between 1500 and 2000 the rest of the day it will be closed. We will put up posters once we have confirmation of the go live date this will be backed up with announcements at the stations.