Blimey! I was at Clapham Junction last month scoping it for possible shots of 61306. Took some photos without any problems whatsoever. Either I was lucky or you were unlucky.
Have only had real hassle at two stations. Once at Chorleywood during a Steam on the Met event back in the 1990s. One member of staff started ranting that I could not bring a dog onto LT property. Complete nonsense of course but he went on to say that if I did not leave the station he would throw all the photographers off the station. To avoid further confrontation I left the the London bound platform and went to the carpark. As the train approached, I stepped onto the Amersham bound platform, took my picture, gave the guy a cheery wave and went on my way. This was the only time I had any hassle during any of the Steam events. The other station was Arlesey a few years ago. The chap in the booking office was a miserable old git at the best of times but seemed to reserve the worst of his attitude for spotters. He'd make announcements over the tannoy and if you ignored them, he'd leave the booking office and come on to the platform to confront you with all manner of nonsense.
I find Arlesey a depressing place at the best of times, so no wonder the booking office person was miserable.
It always smacks as strange when staff makes a scene at those sort of stations, as once the booking office closes the place is left to its own devices, at which point anything goes.
We had a similar encounter some years ago with a rather obnoxious woman at Wadhurst. The conversation essentially concluded with her being told that our activities were continuing whether she liked it or not, and perhaps she might instead attend to the increasing queue at the booking office window, which did do the trick, albeit with a huff.