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.
No point in discussing security from a terrorist point of view - already know there are a zillion replies trundled out in such cases, some as spurious as some of the objections, in fact. Simply different and irreconcilable opinions. Anyway, since the authorities in question own the sites I'd have imagined they had some rights in the matter, but obviously not...