A lengthy BBC article by a SME, Lorenzo Vidino, which tries to describe and assess the problem many countries face.

For the UK he writes:
The head of the UK security service MI5 said this week that fewer than expected of the 800 Britons who joined IS had returned recently and that at least 130 had been killed....The UK Home Office, for example, disclosed last year that of the 400 British foreign fighters who had returned from Syria and Iraq, only 54 were convicted.
Link:http://www.bbc.co.uk/news/world-41679377