Let's take a look at two things:
First, a statement by a local prefect (pdf) about the local French maritime security plan.
The emergence of new maritime risks, and in particular immigration by sea
to the United Kingdom from the coasts of Hauts-de-France and the upheavals and tensions linked to Brexit. In a context of fair sufficiency of emergency means of
mutual support and solidarity between sea rescue actors is the strength of our ORSEC maritime system to be able to face new challenges;
So, while the verbiage isn't super clear, they seem concerned about effecting sea rescues in the context of migrant crossings.
Second, regional press coverage about a sinking in which 27 migrants died:
The general gist of that article seems to be that there is concern that the French navy was insufficiently diligent in providing aid.
That day, in its press releases, the maritime prefecture indicated that many means had been engaged to provide assistance to the shipwrecked, including the Flamant patrol boat of the French Navy. But from what time was he actually sent to the area? To our knowledge, the Flamingo intervened to rescue the survivors on November 24, after being alerted by a fishing vessel to the presence of bodies off the coast of Calais, i.e. after 1:30 p.m., notes Me Agathe Quinio and Emmanuel Daoud, lawyers for the Utopia 56 association, which filed a complaint for manslaughter and failure to provide assistance.
To get back to the question:
Furthermore, are such crossings not equally prohibited under French law as they are under British?
No, why would they be? Human trafficking is illegal, but merely crossing the Channel isn't illegal in itself.
Once the navy is involved why does it not apprehend as criminals under French law rather than “escorting” them along to their destination?
One can disagree with illegal immigration taking place and still avoid calling them criminals.
Truth be told, European countries are still very much feeling their way in how to handle sea crossings for the purposes of illegal immigration. Greece and Italy are the subject of frequent criticism during drowning events. While refugees aren't all that popular, neither is the prospect of letting people drown, which the Daily Mail seems to rather downplay. France has limited inherent self-interest in keeping refugees from leaving France to go the UK and it will require better negotiations between the UK and France to arrive at a clearer policy, rather than just blaming France willy-nilly like this question.