The other thing is once someone takes up arms for a foreign entity, are they still citizens of their supposed homeland (the UK in this case)? Should they be stripped of their citizenship upon joining a foreign army or terrorist group? I think they should. Not sure what UK law is.
Under terrorism act which is in the process of being further updated, joining or fighting as part of a foreign army is not allowed & against the law in the UK. But, each case will be treated individually.
A Kurdish born British citizen joined Peshmerga to fight IS. —http://www.mirror.co.uk/news/world-news/kurdish-born-brit-battling-isis-go-4087596