As for the £200 thing. I know I "floated" this idea but I can't try to evaluate the idea I mentioned without it sounding like a joke. But if I try really hard and set aside my reaction to it: if it really did work than £200 a handful of times per year is nothing. I didn't realize just how few cases of terrorism there are every year, see the cousin comment. It's like, 13 people killed since 2010. How many people do you think are reported multiple times by multiple agencies and citizens? This expense would not even be a rounding error on a rounding error. (But I just can't get over how absurd it sounds.)
I was extrapolating to the more recent events where there were several killers but it applies often to the case where there is a lone wolf because often there is someone else who enables them, perhaps not enough to pin a formal charge of conspiracy on them but close.
Presumably he has to make some plans and if he does that with someone else then they are both guilty of conspiracy to commit a crime. If the police had the resources to follow this up (physical boots on pavement type resources that is) and the courts had the time they could follow this up and have a chance of preventing the planned crime by using the laws we already have.
And perhaps your £200 idea could even work. Not everyone who gets frustrated, angry, and violent is a committed radical. The problem is that we all want this problem to be fixed without it costing us any money, time, or effort.