In the modern world we learn to respect each other's customs.
People who swim in public pools customarily wear bathing suits, not clown costumes.
There is nothing immodest about wearing a bathing suit at a public pool. It is accepted behavior and quite proper.
Men at the pool do not mistake the women who dress according to the prevailing norms as sexually provocative or immodest.
Yes, Ultra Orthodox Jews do not participate in mixed swimming for reasons that only they need to know and to explain to others if they wish to.
So too should be the case for Muslims in France or England. If they wish to act outside of the norms of society, they may stay home and do so and explain their decisions to their group's own members.
By imposing on others a practice that appears to be from the Dark Ages (calling it medieval would be anachronistic), Muslims are mocking themselves. They are not preserving any meaningful cultural or religious values.
See the story, which will continue to gather coverage in the media for another two or three weeks and then fade away, as the summer comes to an end.