Fishy,
If a serving fire safety officer, (or worse), was using their primary employer's name and reputation to gain work and/or credability then I would find that wholly unacceptable. However, using experience gained on fire safety matters is not wrong or unethical. Many professions, such as doctors, lawyers and H & S use their accumulated experience to do private work. It seems to me that there is a little whiff of professional snobbery here. The higher up the food chain you go the more you 'can be trusted' to maintain the chinese walls that allow, for example, partners in legal practices to represent both sides of a dispute. Additionally as employment practices become ever more fragmented, with part-time or fixed term contracts proliferating, how is your position tenable? The issue of conflict of interest from a legal point of view only really arises if the individual has a statutory enforcement role and a privately remunerated advisory role to the same client. (Although the position of approved inspectors in the building proffession doesn't seem to have a problem with this!)
On Kurnals point of fire authority's and trading companies, look at EFA(T) Ltd. (clue Essex Fire Authority (Trading) a wholly owned arm of Essex Fire Authority with links from the Authority's home web site advertising fire risk assessments on a commercial basis. The legal fig leaf of this being an arm's length trading company is really a bit of fiction. If this is not trading on a publicly funded reputation, I don't know what is?