We have border controls, don't we?

There seems to be a real danger that some Brits who have gone to Syria or Iraq to fight with the hardline Islamists will come back to this country and seek to cause atrocities here.  But surely we must know who these people are?  And given the difficulties and delays which we all face in trying to get back in to this country when we go abroad, we ought to be able to intercept these people and either deny them entry, or hold them in detention centres until they can prove they are not a danger to us.  Let's at least get something positive out of our opting out of the Schengen Agreement, eh?