Eventually I got a reply from her, which included a long letter from Vernon Coaker, Minister of State at the Home Office. I have uploaded the complete letter, but here are the relevant portions:
With regards to Mr Rothwell's concerns relating to photographing police officers, there is a misconception regarding the new section 58A of the Terrorism Act 2000.Now I would argue that blogging is legitimate journalism, so I can take pictures of the police. I carry this letter with me - please feel free to download it and use it, or pester your MP for your own copy.
I would like to confirm that a person carrying on legitimate journalistic activity (including taking and publishing photographs of a police officer in the context of responsible journalism) would not be caught by this offence.