Take a look at the link posted earlier showing that daesh are having a problem with Foreign fighters getting pissed off and abandoning their posts / deserting, to the extent that they've created an internal security force and executed a hundred of them.
So any UK born fighters fighting with Daesh who want to come home face the first barrier of being shot by daesh, followed by a second barrier of the likelihood of being locked up for a long time if they openly come back to the UK.
The result being that they're more likely to stay and fight for Daesh.
So a policy of locking them all up on return would be aiding Daesh's efforts to keep their UK volunteer fighters fighting rather than abandoning their posts and legging it back to the UK. It gives them no way out.
The best way of undermining them is to stem the flow of foreign volunteers, and get as many of them that are already there to leave when they're sick of it.