According to this article there is a UK law which may be used, but It's not what the Geneva Convention says. There is also a jurisprudence against it, if I understand well, as also cited in the article.
So, at least according to international law, they have no obligation to apply in France.
As the article states:
"OUR VERDICT
Incorrect. The UN Refugee Convention does not make this requirement of refugees, and UK case law supports this interpretation. Refugees can legitimately make a claim for asylum in the UK after passing through other “safe” countries."