So someone boards at Stevenage, revenue approach them for their ticket, the response is “I go London”, which gets repeated multiple times in response to every question asked, you tell them they’re going to get turfed off at Finsbury Park, which is the next station. Upon arrival there they walk away with a smirk on their face, straight down the (ungated) spiral staircase down to LU for the next stage of their free journey. It wouldn’t make any difference if there was a gateline as they’d just push through, in the same way they probably had at Stevenage.