There've been a couple of articles in Private Eye about the pilot, the late Mr Ibbotson, who did not have a commercial pilot's license and was flying under a scheme (can't recall the name) under which a pilot can take a passenger provided he (the pilot) was going on that route anyway and that the pilot is not paid for the flight. Evidently you have to fly a certain number of hours to retain a private license. Flights obtained in this manner are, surprisingly frequent and iirc there are a number of markets on which these flights can be found; they can be used as a cheap means of flying.
Anyhoo, it appears that a number of the flights surrounding this tragedy were of this nature. Surely not the ones that "Football agent Willie McKay, who brokered the deal, has said he and his family arranged and paid for [these flights].", though.