As the UK currently has no privacy laws, photographers are usually free to take pictures wherever they wish as long as they are on public land, or when on private land with the permission of the owner.
Without permission, one commits an act of trespass. Until 1st June 07 this act of trespass was a civil offense, however as of this date, the offense changed to a criminal one, punishable by up to 12 months imprisonment or fine if the trespass happens in certain places. The list of places include Royal Palaces.
What does this mean in practice for those who do a lot of street photography work? Well as long as you use your common sense and research the areas in which you are working then you shouldn’t fall foul of the law. Common sense is something that we should always use. Take into account your location, the people you are photographing and any sensitivities that may be current due to media reports or even world events.
Safety is paramount really, you don’t want to recieve a criminal conviction over a trivial situation that escalateed to a point of confrontation with the law.