@ David Harper
Outbound domestic and international passengers mingle in the US, sure - but inbound international passengers in the US, including *all* transfer passengers whether their next destination is the US or elsewhere, must each pass through immigration before they can transfer to any other flight - domestic or international. In the UK, transfer passengers do not pass through UK immigration if their ultimate destination is not the UK. Thus, if domestic and international passengers mix in the departure lounges, and swap tickets, an inbound international passenger from another country can fly to somewhere else in the UK without passing through UK immigration. The solution chosen for this is for everyone boarding domestic flights to be photographed when they enter the airport from landside in the UK, or when as a transfer passenger they have passed through immigration, so that each person who attempts to board a domestic flight must have a photograph proving they've been cleared by UK. Clear? :)