Well, firstly, I am not posting as a vicar/reverend/deacon/bishop - I'm not one. So what I say in this capacity is MY interpretation, and I am NOT a representative of the church.
However, it is standard for an employer to have certain expectations when you are speaking as an employee. When I used to work for a very high-profile employer, one of the things they impressed on us was that an email from a work computer was as good as using headed notepaper, and as such, anything we wrote from the computer had to be regarded as 'representing that organisation'. Posting on a forum like this, saying 'I am an employee of XYZ and I think ....' could be sackable, because it was not authorised.
According to the Clergy Discipline Measure, misconduct may be alleged for four things, the fourth of which is "engaging in conduct that is unbecoming or inappropriate to the office and work of the clergy."
Define that as you like. It is not talking about conduct whilst in church, it is ALL the time. And as such, I think could cover 'unauthorised preaching.' I'm not saying just about WAS doing this, simply that if posting as a rev you have to be careful NOT to do this.