The Security Services were pretty much useless at preventing attacks. Despite their collusion in murder, use of inhumane and degrading treatment, internment and fitting-up numerous unfortunates innocent of everything but having an Irish accent. Or rather, because of these things, they were useless as they had no support in the community in which these attacks were planned.
Oh, but mass surveillance is going to be much more acceptable than hooding and beating the crap out of people, isn't it? No possibility of alienating an entire community just by spying on them, surely? Until people get fed up of the police knocking on their doors because they looked at the "wrong" website or have relatives in the wrong part of Pakistan and have been flagged as one of the many high number of false positives that are inevitable. Though of course the security services have mended their ways since the 1970s - no possibility of torture or political cover-ups since then - so those people have nothing to fear, right? Well, maybe there is a chance of history repeating itself,
Where there's acknowledged injustice and no interest in addressing it, you're bound to get terrorism. Much cheaper and more productive to fix the problems. Apart from that, the truly dangerous will stick out like a sore thumb when the majority are happily leading their daily lives without a sense of fear and oppression. How does mass surveillance contribute to that objective?