I've puzzled that return them to France suggestion a lot, Doodledog.
I don't know, but maybe people think that by so doing, the incentive to make the dangerous sea crossing will be removed? So they won't land up here costing us money. Let France deal with the consequences, and take the blame, because by turning a bit of a blind eye to their coming across to us, this kind of implies that France isn't bothered about destroying the business model of human trafficking networks which is where the problem starts.
Basically, it is often perceived that France isn't playing the game. She is just shifting the responsibility in an ineffectual, weak way.