Actually, it's been suggested on Sky news this evening that the third scumbag (to be named) was stopped by police in Italy on his way out to the middle east in the last year or two. Guess what he apparently told them when questioned? 'I'm going out there to fight'. 'To be a terrorist' I think were the words reported. And then he ends up here! This isn't the first time I've heard stories like this. It's not like scumbag number 1 made any effort to hide his extreme views either, albeit in a different format. So I have a feeling that, at least in some cases, there wouldn't be too much work involved.
Where this is reason to believe that British citizens pose a risk to this country in relation to their activities while abroad, some level of monitoring is required. Which I'm sure is already taking place - afterall, we have numbers.
Personally I like the suggestion that anybody with dual nationality be stripped of their British passport if leaving the country to fight abroad. They can't have the best of both worlds any longer. The stakes are too high.