Photographers Rights

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.
With some countries having stronger privacy laws, UK snappers looking to commercially exploit images of recognisable people snapped without their consent may find international clients unenthusiastic unless a model release has been obtained.
There's also a remote chance that photographs of people in public places may be subject to the Data Protection Act, but that's pretty unlikely if there's no other identifying information accompanying the image.