"Bicycles will be removed" - legal?

Hi,

Just a trivia question. I've tried searching here and on other sites and encountered ambiguity.

Do people who post signs that threaten removal of locked bikes from railings actually have a right to confiscate property?

I know councils have powers to remove bikes which are obstructions and the police can do pretty much anything under prevention of terrorism. There are lots of signs around that look like they've been stuck there by a building owner just looking to keep their property looking sharp, but what legal basis do they have to make off with someone's bike?

It seems to me that the majority of these signs are in Westminster, who are cunts as regards bike removal, I've had arguments with the street wardens (thugs) about what their legal rights are to remove private property and they are separate from road safety who do a good job as far as I can see, Westminster also seem to have the smallest number of bike racks of any borough and seldom by the shops or facilities you might want to use.
There is an explanation that I am aware of for some of these signs, Westminster has the largest number of remaining original sand cast iron railings of any borough, as people will know these were mostly removed during WW2 for metal for the war effort except where they protected a drop as they often do in Westminster (a very real danger in the blackout), now being cast iron these are hard yet brittle, if you lock to them a thief can easily break the railings to get your bike, leaving the railings owner with an extremely expensive listed railing repair job.
So that's my understanding of some of the issues at least in Westminster...