I can only speak for America, not England. The first woman to ride astride at Madison Square Garden’s National Horse show (the most prestigious society show at the time) was Eleanora Sears in 1915.
Belle Beach was a preeminent riding instructor of the time, and she predicted that women riding astride would not catch on. I think it was she who stated that women’s thighs were too round to be able to grip a horse correctly astride.
By the time Eleanora Sears paved new ground at the Garden, the aside or astride wars were already being waged in magazine articles and books of the day. So appearing in a Point to Point astride in 1924 would have been cutting edge, but no longer scandalous.
As an aside: Lane-Fox was the name given to the style of park saddles with a cut back pommel. We still call saddle seat saddles “Lane Fox” today. I believe they were first made in England by the E Jeffries company. I do not know if they were designed by or for a man named Lane-Fox. A cut back pommel was seen on side saddles decades before the design became popular on park saddles. They did not reach popularity until America until the 1920s when Lonnie Hayden had one imported or made for the five gaited stallion Chief Of Longview.