I have used Southeastern's Sidcup line for twenty five years, the first twenty two as a peak hours commuter, the last three as a predominantly off peak traveller three or four times a week. My local stations Sidcup and New Eltham are gated, Sidcup gates seem to be manned for much of the day, New Eltham's less so. New Eltham has gates in the station building but no gates on the stairs where most passengers from down trains alight.
The next three stations, Mottingham, Lee and Hither Green, are ungated. There may be occasional ticket inspections there, although I have never observed them from the train.
I cannot remember the last on board ticket inspection I witnessed on the Sidcup line. But I am pretty sure it was well over ten years ago, probably longer.
I have also witnessed passengers doubling up to go through the gates at Charing Cross without tickets. The gate staff seem to ignore it. I do understand the risks of challenging potentially hostile fare evaders. I once asked a chap on the gates about this just after this happened and he just shrugged his shoulders.