A disproportionate number of examiners also teach at well funded public ( ie British for private) schools.
Cheating in recent years has been exposed at Eton college and Winchester school, both expensive Public ( ie British private) schools.
Examiners who worked at the schools were found to have shared questions on the exam with other staff and used it to coach students.
The successful cheating occurs with adult help from corrupt examiners who also teach and use the inside knowledge to benefit their students.
The scale of this is unknown but recently some evidence has emerged.