No it isn't. Never check with anybody or ask permission unless you want to gain access to a gated station and you don't have a ticket. However, if you are intending to be at a station for a long period, especially a station which doesn't get many enthusiasts it can't do any harm notifying staff of what you are doing.
(Note the use of notifying them, not asking their permission)
I know taking photos of trains is a fairly trivial thing but bringing up “the current security scare situation” does make me angry. Using security to restrict harmless and legal activities, no matter how unimportant the activity is, is just the thin end of the pretty unpleasant wedge and should be challenged whenever it rears its ugly head.
Go to Paris then, even before the recent issues there it is not uncommon to be questioned by the armed forces or police when pointing an SLR around. The the heightened security situation WILL be an issue whether you like it or not. Getting angry about it will just make you that. It will make YOUR blood pressure rise, but nobody else will care one hoot about your anger. This situation is not going to get better, it is very likely indeed it will get worse, so best see your doctor for some beta blockers.
There are some very very simple steps that we can ALL take.
PRINT a copy of the BTP guidelines before you set off.
Be POLITE when approached by people who do not understand these guidelines and ask them to read them. The amount of times I see vindictive comments on forums makes me ashamed at times. Jobsorths are in the main only doing what hey have been told and being rude and arrogant the their face or on a forum is exactly the same thing. I suspect the vast majority of people who come on here to rant do not have jobs where the public poke cameras around their work place!
The industry does need to do much more to brief staff on the situation. I have been involved in a few conversations with managers who have sent emails around on the subject, politely pointing them in the direction of the ATOC and BTP guidelines.
Network Rail tend to be the worst at this as the station staff they employ are in the main front of house staff and have no real involvement with the real railway and when they have a brief every shift telling them to look out for suspicious activity that is what they do. This is NOT over zealous. The closest I have ever been to an exploding bomb was the one that went off on the concourse at Victoria station. I was there ten minutes before it happened and the window of my office was shattered where I had been sitting. Unfortunately the people on the concourse were less lucky.
The signing in scenario is a bit of a shambles, I accept that, but at the same time I know, as does every single enthusiast who reads forums, they exist. If I jump off a train and see something I want a snap of, i will do so. I also acknowledge that an employee may come and ask me if I have signed in, at which point I will have little choice but to put my camera away. If I was going to a station to platform end I really cannot see why signing in is such an issue, other than to deliberately antagonise staff and allow people to come back to forums and say "I told you so".