Somebody who has already gone through airside in an area of the airport where there are international departures and who leaves the controlled area (i.e. airside) otherwise than on a plane. In principle, you are not allowed to do it without HMRC authorisation. HMRC policies seem to vary from airport to airport. For instance, at MAN T1 a few years back, they would not let me through unless accompanied by a representative of the airline I was flying.