One of the answers is that murder or manslaughter committed by a British citizen abroad is still triable in Britain as a crime.
Where any murder or manslaughter shall be committed on land out of the United Kingdom, whether within the Queen’s dominions or without, and whether the person killed were a subject of Her Majesty or not, every offence committed by any subject of Her Majesty in respect of any such case, whether the same shall amount to the offence of murder or of manslaughter, . . . may be dealt with, inquired of, tried, determined, and punished . . . in England or Ireland . . . Provided, that nothing herein contained shall prevent any person from being tried in any place out of England or Ireland for any murder or manslaughter committed out of England or Ireland, in the same manner as such person might have been tried before the passing of this Act.
(Offences Against the Person Act 1861)
Hence one of the reasons for the concern that the man who beheaded the journalist was British.
The answer is not to "send these people back". If they want to apply for citizenship of another country they can. If they want to maintain their British citizenship they can - but they will be subject to British laws. If the law relating to murder and manslaughter abroad was extended to include terrorism, then if they came back to the UK they could be arrested and charged.
It requires communication between countries and the sharing of intelligence, which is what the world needs. Iraqi intelligence services can provide UK intelligene services with the proof of terrorist acts committed in Iraq, for instance. And surely that is a better way of dealing with the problem of terrorists than just sending depriving them of British citizenship.
After all, do you really think that all Iraqis want IS terrorists in their country?