This happened to me on a flight once. We were taken through normal arrival channels and were instructed to return our purchases to the duty free store, where we were fully refunded before returning landside. An HM Customs Officer was lurking in the general vicinty, presumably to make sure that everyone complied.

Note that this was in EDI where the duty free shops are located in an area that's accessible to both arriving and departing passengers (you need to show your bp to make a purchase).