According to the guardian's article at [1] it appears the system will actually be opt-in. From a leaked letter sent from the Department of Education to the ISPs

According to the guardian's article at [1] it appears the system will actually be opt-in. From a leaked letter sent from the Department of Education to the ISPs: "Without changing what you will be offering (ie active-choice +), the prime minister would like to be able to refer to your solutions [as] 'default-on'" active-choice+ is a set of filters that may be enabled on request. [1] http://www.guardian.co.uk/commentisfree/2013/jul/21/david-cameron-war-internet-porn http://www.guardian.co.uk/commentisfree/2013/jul/21/david-ca...