I love the concluding sentence from this article from the Telegraph. UK passport controls were relaxed because the French authorities were complaining about congestion.
Bureaucratic necessity wins out every time.
All but the most cursory checks were abandoned on passengers on British-registered coaches as they arrived at Dover, Britain's biggest port.
Instead of passports being scanned electronically, border guards checked that the picture matched the holder. It means they were not cross-checked to a computer database to establish if the holder was a wanted terrorist, criminal or immigration offender.
The policy was in place for four years after being introduced when Labour was in power, but never disclosed to Parliament.
It was implemented because the French complained about congestion in Calais caused by backlogs at passport control.