I'm firmly of the belief that if staff employed by a property owner requests you leave, you do so. It is their property and they can at any time request people to leave it, it's a fundamental right regardless of whether anything is banned or not.
I don't want to get buried in the technicalities of this instance as there were wrongs on both sides. But strip it down and basically it is what I said above. An employee of the property owner (Network Rail) asked someone, for whatever reason, to leave their property. The reasonable response would to show a valid reason for your being there (a ticket) and to stop doing whatever made the property owner (if it was a specific action) ask you to leave.
It's pretty clear to me as well there were wrongs on both sides in this instance - I remember talking to Matt not long after this had happened and telling him he could've handled it better. Maybe he's learned from it - I don't know!
On the wider issue, to me, all the TOCs and NR (at senior management level) and BTP have agreed to the enthusiast guidelines. Those guidelines give enthusiasts express permission to photograph at stations providing simple rules are followed - like NO FLASH PHOTOGRAPHY(!!!) - and if those rules are followed then as far as I'm concerned station staff have no right to disregard the wishes of their superiors.
As for BTP - it's clear to me that as long as they do something - however pointless and futile it is (like these endless S44 stop and searches being carried out on rail enthusiasts) - then they are doing their bit for security. Until the penny drops that this action does more harm than good then we'll be stuck with it. It's complete madness, we all know it, but that's the crazy world we live in!