And what happens if they are british citizens born in Britain without a second nationality?
Where do they go? Join ISIS in Syria and Iraq? I'm guessing most wouldn't be interested as while they aren't happy with the way things are here they would not like to be in a nation ruled by ISIS...
They don't go anywhere as you can't make a person stateless

I'd much prefer terrorists rotting away in prison for the rest of their lives than dead or out there in another country.