There was a similar incident a few years back with a railway buff:-
http://www.lynnnews.co.uk/news/local/this-may-look-boring-but-you-cannot-take-this-picture-1-2489174This is what the MET has to say about it:-
http://content.met.police.uk/Site/photographyadviceBasically if captain clipboard comes along and has a go then ask him to quote which rules you are breaking, if they are unable to then advise them that you are more than happy to speak to the police about the matter and prove that you are not up to anything sinister.
Don't get into an argument with them just keep cool and stand up for your rights.
Personally if you have a DSLR around your neck taking photos then its hardly as if you can be labelled as a terrorist, how many people walk around with a mobile phone in their breast pocket making a video as they walk ?? How many of these people are stopped. Likewise dash cams are becoming more common in trucks, are these turned off on entry to dock estates ??
Google earth/street view has made it much easier for terrorists to plan attacks from the comfort of their caves, google street view has given them the option to walk the path with zero risk.
In my opinion removing photo permits has made it worse for port operators as they previously had a system of control for al persons visiting ports for the purpose of photography, now they have people photographing on the perimeters, through fences, from high vantage points etc etc and 90% of those people they have no idea of their identity.