This seems to sum it up I don't think the large ISPs have thought about the legality of this action...
author of 'The Hacker's Handbook' (1980's) Professor Peter Sommer, warns about this and the link below shows this could conflict with the Regulation of Investigatory Powers Act (RIPA): as posted in the News on ISPreview
http://www.opsi.gov.uk/acts/acts2000...1-ch1-pb1-l1g1