divebuddy, I have no idea. However, as I mentioned elsewhere, the German scheme, Hayat, has had some success and is going to open an office here with Home Office support.
The way I see it is this. You have young Muslims who meet radical friends either via the mosque, youth groups or social media, become radical themselves and get sold on the idea of saving the world (well Islam anyway) from the threat posed by the West.
They then go out to Syria and Iraq and sign up with ISIL. As they have no military experience, they get some basic training and then, perhaps, get sent out to fight. since they have been educated and raised over here, they are, despite their radicalism, influenced by their education and upbringing an see and experience things out there which disillusion them. They then want to get out of there and come home.
I don't see a disillusioned young Muslim as being a serious threat to our security or safety, so I doubt that the security services would need to keep track of them full time. And I also think that, having experienced what things are really like out there, at least some of these young men could be used to help prevent others from going out there as well.
However I'm not a complete idiot, and I don't think that all of the ones who return will be disillusioned - some may well threaten the safety of our citizens and the security of our country, so there would have to be some sorting of the wheat from the chaff. How you do that, I have no idea, but I don't think that presuming that all who return are guilty is the way to go.