"flagellation so rife"

Flagellation for erotic purposes was so associated with the English that it came to be known as "le vice Anglais" (the English vice) by the French.  It is certainly true that this fetish was well represented in pornographic material of the time.

The poet Algernon Swinburne (1837-1909) claimed that his taste for flagellation originated in the corporal punishment commonly used at Eton, the expensive boys' boarding school which he attended.  If this theory is true, it would perhaps explain why this fetish was most common in the upper classes, who would have attended similar expensive private schools.