There are also some yellow validators on the paid side of the barriers to the far (high platform number) end of the underpass. Or there were a year ago. It saves going in and out of the gate line but it’s a bit of a walk if you’re not coming on or off a Southern served platform, particularly at busier times.
I’ve done both, I feel more silly going through the barrier twice but more guilty tapping in or out on the validators! (To be clear, when I do it, I’m not guilty of anything other than a legitimate change of ticket type.)