No. Taking a photo in a public place is not an offence. BUT this is a highly contentious area and many people (even some whose job is to enforce the law) seem to be unsure of what the law is. This has led to many cases of professional and other photographers being hassled by the police though they were doing nothing wrong.
Professional photographers have tried to correct the situation by publishing guides to what the law really says and even summary cards to hand out to people to put them right.
One such site is
http://www.urban75.org/photos/photographers-rights-and-the-law.html which includes this quote from Home Office Minister Tony McNulty:
"There is no legal restriction on photography in public places, and there is no presumption of privacy for individuals in a public place.
It is for the Chief Constable to ensure that Officers and Police Community Support Officers are acting appropriately with regards to photography in public places, and any queries regarding this should be addressed to the Chief Constable.
However decisions may be made locally to restrict photography, for example to protect children. Any questions on such local decisions should also be addressed to the force concerned."
That said, the legal position is one thing; what criminals and their families will do when cornered is another matter entirely. Being in the right legally won't stop someone else assaulting you or damaging your gear.
For a more detailed guide, see
http://www.sirimo.co.uk/2009/05/14/uk-photographers-rights-v2