In charities where those positions are voluntary, all positions are generally voluntary. However, I have worked for one with paid everything else and a voluntary Finance Director.
Please note that people who volunteer in senior roles aren’t randoms off the street. In my experience, they are always professionals who are already doing that role, with great success, professionally. So, the (volunteer) HR Director of XYZ homeless charity is also a (paid) HR Controller at XYZ hedge fund. They are skilled, experienced and up to date on all trends, developments and legislation.
Additionally, none of these people ‘come in occasionally’. There will be set times/hours/duties.
‘Volunteer’ is a salary grade. Nothing more. It doesn’t indicate experience, skillset, dedication or the amount of time invested. It certainly isn’t indicative of professional accountability.
As stated upthread, the majority of our justice system is in the hands of volunteers (magistrates). They don’t exactly come in occasionally with accountability and the fact that most other roles in the courts system are paid doesn’t impact on that.
Please also note that every registered charity in the U.K. is run by a Board of Trustees, all of whom are voluntary.