One huge difference between Orwell and Syrian free-lancers was that Orwell was in fact fighting for the recognized government of a nation state and under its legal authority. This technically violated the unenforceable Foreign Recruiting
Act of the time, but would not be a violation of the current Terrorism Act.
It would have violated many other laws of the time if Britain had been at war with Spain, as they are with Al Qaeda, but the countries were not at war.