The "Law in Action" series on BBC Radio 4 recently aired a very enlightening program titled "Conflicting Rights". The middle section of the transmission (about 10 mins in) has an interesting feature on photographers's rights in the UK and how the law affects one taking photographs in the street. They interview an ex-pro photog turned lawyer by the name of Rupert Grey who divulges some useful insights. They also get a statement from the Association of Chief Police Officers (ACPO), which goes like this:

"Police officers may not prevent someone from taking a photograph in public unless they suspect criminal or terrorist intent. Powers to stop and search are strictly regulated by law. One an image has been recorded, the police have no powers to delete or confiscate it without a court order."

The broadcast can be heard here.