One is accustomed to being assured that Airport Security ensures that you show a Passport in order to check in, then show your Boarding Pass (but often NOT your Passport) to get through Security to Airside, but finally show both your Passport & Boarding
Pass to get on the Plane.
On Fri 9 Jan 2009 I was flying with American Airlines from NY JFK to London. When we boarded the plane, all they did was check the Boarding Passes. They announced over the PA beforehand they would NOT be checking Passports.
So that means anyone in the mixed (Domestic/International) terminal could have exchanged Boarding Passes and got on that plane.
When arriving at London Heathrow you don't have to show your Boarding Pass, or prove which flight you came in on, so as long as you have a Passport with a matching Photo you can gain access to the UK.
So nice of the USA, who make us jump through hoops to gain access to their Country, to let anyone board their Aircraft and travel to Europe.