[user104658]

It's a tough one really. If they're not originally British citizens (born and raised) then they shouldn't be accepted back into Britain for obvious security reasons. Sending out groups to specifically hunt them down in other countries seems a bit... err... "war-crimey" I guess.

But then there are those who are British born and raised - what to do there? You can't really say "don't let them back in", they are our responsibility, by not letting them back on UK soil... they don't just evaporate... they are still out there in the world free to main and kill in countries that, supposedly, we are trying to help liberate from these people. And as the UK does not have the death penalty - the ONLY obvious answer is that they should be brought back into Britain and then prosecuted for treason and, presumably, murder and other crimes.