If I could, I would prevent their reentry into their ‘home’ countries. These murderers don’t deserve to live among the civilized.
Freshly minted jihadis fly to Turkey, and then take a taxicab across the border. One could also go to Jordan, which has a very long and essentially unguarded border with Syria. One could cross from Lebanon as well. They will have enough support from fellow jihadis in those places; muslim border guards will just look the other way. A foreigner in a 3rd country is nobody.
Short of being wounded and needing care, nobody in UK or other home countries would ever know where these travelers have been. But even if a jihadi chooses to cross the border officially, with a stamp in the passport ... it's still his human right to do so, as long as both countries do not object. Fighting as a mercenary - that is illegal. But mere visiting is fine, especially if the visitor somehow relates to Syria.
Bingo:
Don’t let these animals back into your country.