The only places I've had problems are Liverpool Lime Street (that was worst, searched and threatened with arrest by police if caught again, just for walking down the platform to see the loco coupling onto the front of the XC service), Leicester (only once, and to be honest I don't even know if he was staff, at best he was a cleaner), St Pancras (a couple of times between LCR taking over security and the move to the interim station, although thankfully not on the last day), Brighton (not me specifically, but someone tried on some enthusiasts taking a photo of a slammer, but gave up against weight of numbers since we'd just got off the "Last Thump" railtour) and Bromley South (most amusing thing was that I was photographing a slammer as it came in that I was going to catch back to Victoria, and he followed me as I got on the train to keep telling me I couldn't take photographs, running down the platform to catch up with me).
Ironically, every single time I've had problems I've been taking one or two photographs whilst waiting for a train, each time I've had a valid ticket, and on two occasions I had to walk away from the situation to board the train I was waiting for. Also, not once has security been mentioned, not once have I been told I needed to sign in or get permission, it's always been along the lines of photography not being allowed on that station, which actually is a downright lie (there is not one station in the country where photography is banned, or even where permission is required, and that includes the international stations other than the security areas, infact outright photography bans or requirements for permission are very rare at any station in the free world). Whilst I don't make a habbit of hanging around station photographing, the times I have done it I've never had a problem, despite sometimes being there for over an hour, often down at the end of the platform.