From Privacy International's Know Your Data Campaign:
"Many of the companies that supply Britain’s communications services – the landline services, cable companies, mobile operators and Internet Service Providers – are accumulating a vast amount of personal information about their customers. This "communications data", which is currently stored for up to seven years, may relate to all the calls you have made and that you received, who you are in contact with, the geographic location of your mobile calls, the emails you have sent and which you received, the websites you have visited, the television programmes you have watched, personal financial data and other personal information about you and your family. Combined, this extraordinary array of data creates a comprehensive dossier on the contacts, friendships, interests, transactions, movements and personal information of almost everyone in the UK."
See also the Foundation for Information Policy Research website, Scrambling for Safety 6, for links to related government and public interest documents. In addition, also see this article, UK gov seizes data on 100m calls, 1m users, a year.