If they are British born, then a special prison where do they not come into contact with anyone else.
Nice idea - but wouldn't it become an even hotter hot-bed of murderous Islam?
More to the point - how long could we keep them there? as long as they remain alive would be good - but that will cost us taxpayers £millions - Millions to keep them fed and watered (and guarded), £millions of taxpayers' money going up in smoke fighting court cases brought by civil rights lawyers, and £millions more policing the demonstrations and protest marches by civil rights activists claiming we are turning into a fascist (and probably racist, homophobic and xenophobic too, according to them) society denying these poor misunderstood souls their human rights.