From what I have seen and experienced this is a grey area in law, for example I was once interview in the street for a vox pop news item. I was under the impression at the time that such interviews required explicit permission to broadcast, and I had not even been asked for name etc; however within an hour I was on local news. On the other hand there have been a number of complaints upheld by the Broadcasting Standards Agency where members of the public have complained about their starring role in docusoaps (perhaps surprisingly not Airline). I believe that the only grounds for complaint is if the material has been edited unfavourably, and the above waiver notice is probably there to prevent professional actors claiming a fee for being in the background.