Catching up on my reading I found this early December 2017 article by Shashank Joshi (ex-RUSI), written after the brand new Defence Secretary, Gavin Williamson call the day before:
A dead terrorist can’t cause any harm to Britain....I do not believe that any terrorist, whether they come from this country or any other, should ever be allowed back into this country. We should do everything we can do to destroy and eliminate that threat.
Link to Joshi's article:https://www.theguardian.com/commenti...ary-not-simple

Link to the report on the Defence Secretary comments:https://www.theguardian.com/politics...vin-williamson

The big snag for the UK is that IIRC four hundred suspected fighters have returned already; a figure officials now cite. Only a fraction have been arrested and prosecuted.