Next time, look out for the Northumbria Police desk - behind which may or may not be standing a Northumbrian Policeperson (generally plain-clothed). It all has to do with the "Common Travel Area" as far as arrivals from the Republic of Ireland, the Isle of Man and the Channel Islands are concerned. Because passengers coming from those points of origin did not require to pass through passport control on entering the UK as their countries were part of the long-established CTA, the only way to monitor them was for them to be routed past police officers. The requirement was extended to cover arrivals in Great Britain from Northern Ireland under the Prevention of Terrorism (Temporary Provisions) Act 1990, so I believe.