Most of them have claimed asylum in the UK because they are wanted by their home countries. Being deported home is not going to be a picnic and they won't be roaming the streets.
Kick them out and make them understand that this is their only chance at maintaining life. Make absolutely sure that they understand that if they return as illegals they will simply disappear. I guess they could just make them disappear now and skip a step or two. I defiantly wouldn't miss them.