The point is.. they have no business in asking you for your credentials UNLESS they have reasonable suspicion (if I understand UK law correctly). In other words, the fact that they're asking for your credentials means they suspect you are a criminal or are engaged in criminal activities. And that's what is bothering the photographer.

If he was covertly trying to take pictures with a cellphone or compact from behind some bushes , at least then I could understand the officer asking some questions. 

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