Getting on a plane in EU with EU passport is easy. Getting off the plane in London will get you to a proper security check. They would have caught him there, hadn't he declared his syrian nationality.
Hell, most of the times I've flown between European countries, nobody even looked at my ID. If somebody did, it was usually check-in staff, so there would have been nothing preventing me from checking in under my own name and then handing my ticket to somebody else.