HMRC says its non-executive directors are:

senior business figures from outside the department who bring a diverse mix of expertise and skills from across both public and private sector.

Richard Murphy of Tax Research UK gives his view:

Who should be running HMRC if it isn’t to be the Big 4 and ‘senior business people’?

HMRC should be run by people who are committed to the social value of tax, not by people who believe it is best avoided