1. It is not ever illegal to take pictures in a public place in the UK, irrespective of what is going on.
2. Children have no more right to privacy than an adult does when in a public place.
3. Any member of the public has no powers to demand ID from anyone under any circumstances.
4. Forcible deletion or removal of images or destruction of film from your camera is an assault.
5. Detaining you and taking your camera would constitute an unlawful imprisonment or theft and both would include an assault.
6. Even child protection officers (CPO's) have no right to stop you or demand ID
http://www.chapterthirteen.com/index.php?option=com_content&task=view&id=66&Itemid=56
Street shots
If you're on a public right of way - such as a public pavement, footpath or public highway - you're free to take photographs for personal and commercial use so long as you're not causing an obstruction to other users or falling foul of anti-Terrorism laws or even the Official Secrets Act (frankly, this one is unlikely).
People and Privacy
UK laws are fairly vague when it comes to defining what constitutes an invasion of privacy, but while street shots should cause no problem, you might get in hot water if you're strapping on colossal telephoto lens and zooming in on folks stripping off in their bathrooms - even if you are snapping from a public place.
The key seems to be whether the subject would have a reasonable expectation of privacy - a statement that seems vague enough to keep a team of lawyers gainfully employed for some time.
Photographing children
There are no laws against taking photos of children,
Deleting images
Security guards do not have stop and search powers or the right to seize your equipment or delete images or confiscate film under any circumstances. In some circumstances, the police may grab your film or memory cards but they are still not authorised to delete any images. After all, if you've committed an offence the images would act as evidence, and if you haven't broken the law, the images are innocent.
http://www.urban75.org/photos/photographers-rights-and-the-law.html
MP and Foreign Secretary David MilibandThere is no legal restriction on photography in public places, and there is no presumption of privacy for individuals in a public place.
http://curly15.wordpress.com/2008/04/19/miliband-on-photography/
http://www.sirimo.co.uk/2009/05/14/uk-photographers-rights-v2/