I think this would be covered by the same rules of taking pictures in a public place that the press adhere too.
Posted from the ACPO directive online:
There are no powers prohibiting the taking of photographs, film or digital images in a public place. Therefore members of the public and press should not be prevented from doing so.
We need to cooperate with the media and amateur photographers. They play a vital role as their images help us identify criminals.
We must acknowledge that citizen journalism is a feature of modern life and police officers are now photographed and filmed more than ever.
Unnecessarily restricting photography, whether for the casual tourist or professional is unacceptable and it undermines public confidence in the police service.
Once an image has been recorded, the police have no power to delete or confiscate it without a court order.