There is an assumption that Britain leaving the EU would not have any affect on other bilateral arrangements, That is a very dangerous assumption.
The French plainly would like the migrants at Calais to skedaddle across the channel, but at the moment they can't pack them off UK-wards.
Who is to say what their attitude was if the UK was no longer in the EU.
There is a widespread, mainstream view in France that the problem is at least as much Britain's because, as it is not in the Schengen zone, the migrants are stuck. This is not, in my view, a very logical way of looking at it, but it is what they think. If Britain were still not in the Schengen, and not even an EU partner, then it's not hard to see attitudes hardening.
Having said all that, I hope we'll see some more positive pro-EU messages from the Prime Minister soon. There is something unwholesome about this migrant-mania. The man who nearly split up the UK may soon find himself in charge of the one-party Tory state of England ....