It seems that Clergy have no ‘office-holder’ option when they fill in their census forms.
According to an announcement from St Albans diocese (scroll down), and no doubt others:
The Archbishops’ Council says:
(a) We very much regret that The Office for National Statistics has not provided a box that allows non-employee office holders to respond accurately.
(b) Clergy will have to decide whether to tick employee or self-employed, neither of which is accurate.
Considering all the fuss that has been made about the status of clergy as office-holders, this has to count as a #fail for the Archbishops’ Council.