Thats ok then
Under the Human Rights Act, individuals are guaranteed the right to privacy surrounding their communications other than if a public authority, such as the police, believe it necessary to interfere with that right "in the interests of national security, public safety or the economic well-being of the country, for the prevention of disorder or crime, for the protection of health or morals, or for the protection of the rights and freedoms of others".
Ah nice and broad then. You could envisage ANY scenario within those guidelines which the police could use grounds for interception. Why don't the government just have a law which states the truth for a change - "We can intercept anything you do, and you cannot do bugger all about it". Would save a few trees in the process to boot.