The Metropolitan Police began offering such a service in beta in August 2008 which now goes down to sub-ward level (you access levels of detail by zooming in on the maps).
Police have used mapped crime information internally for some time when allocating resources. Please note as maps for different forces were created locally, the level of detail can vary.
The Met's version is here:
http://maps.met.police.uk/
The Home Office statement is here:
http://press.homeoffice.gov.uk/press-releases/online-crime-maps
