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.
Syria objects. Additionally, you can't have a human right that is contingent upon permission be granted. Simply put, you have no inherent right to enter another country.
Fighting as a mercenary is the operative phrase here. If the countries from which these “fighters” hail are enemies of the recipient countries, they can be charged with a crime.