Have you noticed that the UK press is up in arms about the migrant hoards truck-jumping at Calais; the British people are incensed at the traffic jams on the motorways and the failed attempts to go on holiday, and yet the British government refuses to take more than a very mild line with the French?
Darkling hoards despise fences
It's because the French have granted us a generous concession. We get to put our immigration officers on their side of the border so we can check entitlement before someone actually enters the UK. Without this concession any person wanting to enter the UK would be on this scepter'd isle before being asked for papers. The hoards gather at Calais only because we are allowed to stop them on French soil.
Anytime we piss the French off they could withdraw this concession, tell us to take our bobbies home, and we would have to deal with the immigrants after they were already in the UK. That's a two year process - during which we are obliged to feed and house the applicant for admission.
So don't knock the French. They have no obligation to police people leaving their country.