Article 19 of UNCLOS says that if a foreign vessel enters another country's territorial waters it will be considered to be prejudicial to the peace if it engages in the loading or unloading of any person contrary to the immigration laws of that country.
Migrants cannot be returned to French shores, unilaterally, and without prior agreement with France.
In 2019, France and the UK agreed on a joint plan to tackle irregular migration which does provide for cooperation, but it DOES NOT allow one country to bring people rescued in the English Channel to the other country's ports.
This has all been explained here many times.
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