This Breitbart report http://www.breitbart.com/london/2017/03/28/hundreds-islamic-state-volunteers-returned-britain/ should scare the crap out of you. Here's an extract, but do read the whole article:
'Security sources have told Sky News that more than four hundred British jihadists who travelled to the Middle East to fight for terror organisations like Islamic State and the al-Nusra Front have returned home, and most have evaded the authorities.
Former Scotland Yard Specialist Firearms Officer and author Tony Long told the broadcaster these fighters could pose serious problems for the British authorities.'
If the primary duty of the State is the safety of its people then the British state has been derelict in performing that duty.