Just to be clear about the background and implications of this, as many people reading this are not UK citizens, the 'paranoia' is not imitative, it does have a real foundation.
There was a terrorist bombing, now designated '7/7' for obvious reasons, in one day on three London Underground trains and a bus, with many killed and maimed. There have since been several foiled attempts in the UK, and there was also a bombing of the railway in Madrid, again with many killed.
Subsequently, in a tragic case of mistaken identity, armed plain-clothes security officers were trailing a Brazilian electrician, and followed him on to the Underground. He was not a tourist, and there was speculation raised about whether he might have been lacking a work permit, but most likely he thought he was being pursued by muggers.
He ran.
He was shot dead to ensure that he did not let off a (supposed) bomb on a crowded subway platform.
The rights, or otherwise, of photographers did not come into it.
Just so that we realise why the security procedures are what they are, for better or worse.