Yes, I know it's fashionable on this forum to attack any sign of authority or of restriction on photograhy, but doesn't it occur to anyone that large cameras and tripods, especially tripods, can be a damned nuisance in various instances, the confines of a tube station being one such. I assume we are talking about an Underground stop? I'm not terribly au fait with London, going there only when unavoidable.
If that was really the purpose of the sign, it's pointless. I'm sure there are already rules in the Tube by-laws against blocking exits and hindering flow of people. One doesn't need a camera to do that. And photographers who do that can be already told to move on according to existing rules.