Culturally, the French are very much more blasé about relationships that would seem very inappropriate to the British.
This. Or can you imagine how that would have played on the campaign trail in the US presidential election? Oh, hang on, they elected a man who talks cheerfully about grabbing women by their genitals, has had multiple women make credible accusations of sexual assult, and who apparently had a habit of dropping into the dressing rooms of semi-naked pageant contestants when he owned Miss Teen USA, which included girls as young as 15, which he admitted to in 2005 to Howard Stern.
Yes, it is interesting to see how this has played in France. Half the references I've seen to it in the French media seem to have been various former colleagues and friends of EM's saying that the relationship existed was testament to EM's 'persuasiveness' and ability to overcome his parents' opposition etc, so converting it into evidence of his savviness/singlemindedness.
I do think part of the reaction in this country is pure spite (good-looking young, powerful man with a much older, less obviously attractive woman, when the opposite is entirely 'normal') masquerading as concern about a teacher having a relationship with an underage boy. (Though I gather they deny anything took place while he was underage...?)