I just read this article too with great interest.
I photograph lots of places in London and frequently see signs that say that the area is privately owned. But many areas do not have signs so you've no idea.
The area next to City Hall is ideal for a sunrise photo of Tower Bridge for example, but apparently that is privately owned, so they could be within their rights to shoo you away.
I've not encountered aggressive security guards thankfully though. I have been told that I couldn't use a tripod though, late one night trying to catch Covent Garden with no one in it. On the same late night walk I took a shot of a privately owned shopping area, but I used a tripod on the pavement outside the private street.
To have an area that is private and to have restrictions that are policed, but to not allow members of the public to see the restrictions is rather unfair.
If the government could put a law in place that forced landowners to have their restrictions available to the public, that would at least make things fair.
It's not a new situation though. Even Hampstead Heath is privately owned and you'll get stopped if you try to put a tripod up. I've seen the Royal Parks Police move on a photoshoot taking place in Regents Park, but they were being rather stupid about it, with light stands, softboxes and modifiers, it was rather obvious what they were up to.
I think in the cases of some land owners, they use the excuse of wanting to stop terrorists photographing potential targets, but for a lot, it's the fact they don't want someone else making money from a photograph of their building or taken on their land, so they charge a fee for photography.
The map, linked to the Guardian article, is worth a look if you ever take photos in London.
Here