Oh, my bleeding heart 
In around 2015, as an experiment, my DS1's school, a top performing comp, did an experiment, as they were toying with IGCSEs. I acknowledge they only did Maths as English would have meant 2 different lots of books.
So with parental permission, DC were entered for both IGCSE and AQA GCSE. The school, being a comp, had to make sure the cohort reflected the school cohort, less able to stellar.
The results were, frankly, shocking, particularly at the pass/fail boundary, and at the A/A* boundary.
The DC statistically did considerably better at IGCSE.
The reason the school didn't than adopt it was because League Tables weren't counting IGCSEs any more. As much as anything because they are not 'nationally moderated'.
Times change. Things move on.
The all-new GCSEs are harder than IGCSEs. So I imagine most private schools will stick with the latter.