One might think the commenters here never traveled outside the UK. The idea that a security guard stopping people from going through a door the wrong direction would be too much staffing demands for a major international airport is ludicrous.

This is exactly what's done pretty much everywhere around the world. From the international departures area after exit passport control if applicable or where duty free is sold there's always a door that leads to immigration and customs with a security guard making sure nobody goes the wrong direction.