I have a very easy solution. I always believe 99% of all problems tend to stem from bad parenting.
Someone joined a radical islamic group? Arrest his or her parents, siblings and close relatives for a bare minimum of 2 weeks. Any of them on benefits? Stop those. Any of them in an institution of higher learning? Expel them. Any of them hold a civil service position? Sack them. Any of them been tax evaders? Collect the taxes owed and add a penalty of 1000% of the amount owed.
Someone has become a terrorist? Excellent, any damage they inflict the reparations will be borne by their immediate family, if they can't pay within 1 week, their properties shall be seized and auctioned off. Not enough? Send them to a labour camp and make them to clean graves, religious sites of every other religion.
The terrorist radicalised in UK? Fine. The mosque or islamic centre will have its assets seized and premises shut.