Posted By Whiteway:
There seems to be a London-wide ATM (Anti-Tripod Movement).
I believe it's separate from the other ATM (Anti-Terrorist Movement), since I've been leaned upon to take my camera off its monopod in a London park (Wandsworth Common). Without its monopod I was free once again to click the camera's shutter.
You could try a beanbag. Not as effective as a tripod obviously but quicker to set up and put away when the long arm of the law turns up.