PFR the idea has some merit as statistics prove recidivism occurs. I would argue that the little buggars that have not been captured likely pose a much higher risk of continuing their nefarious pastimes and despite increasing legislation concerning chemical sales those to make explosives can still be sourced. Even the Anarchists cookbook is still on the internet.
Internment has been used before and would have the benefit of reducing public risk. Better safe than sorry. What we have now could well be the face of modern warfare, we have yet to develop the tools to combat it
http://www.claws.in/images/journals_doc/742067376_MBSushir.pdf

Some argue deradicalisation takes several generations

http://studies.aljazeera.net/en/reports/2018/05/100-percent-defeat-isis-ideology-180508042421376.html

There is an additional problem of quantifying the risk posed by "non belligerent" members. If steeped in the ideology and unrepentant they also can pass their toxic message to their offspring unless they are taken from them. Without witness or evidence how do you establish non belligerence? Why bother? fighting or non fighting they can still transmit their extremist ideology and that in itself is a risk.

As far as the UK is concerned it is an unholy mess. We can forgive terrorists their sins yet persecute our soldiers fighting them by allowing ambulance chasing legal vermin to pursue them in the courts decades after the alleged "crimes"