What countries offer this more forceful and intrusive preventative action? Do they have a greater or lesser number of attacks/threats than the UK? Not necessarily the most important point (there will be many contributions to whatever the answer is) but something to mull over.

In this attack, if he was known and not considered a serious enough threat then why was this the case? What makes a "major risk" compared to a lesser risk?

If they attack our freedoms and we give up our freedoms as a result then they have won. At the same time we never want an event like last night to happen ever again. It's a difficult equation to answer.