I agree that is the most important distinction. The filter which is just coming into force, on BT (what was British Telecom, and one of the largest ISPs) is what's called an 'unavoidable question', where you have to make a choice when you first go online, to be filtered or not (and if so, at which level) so effectively opt-in or out. I don't really understand why people complain about that on free speech grounds. There has been government pressure to set up decent filters, but these are ultimately just private sector ISP-level parental controls. I would personally expect such features to be standard.