FNG
I think you will find that a CPL is nearly worthless without the aircraft operator having an AOC.
It is the lack of an AOC for which flying schools have been prosecuted. This is also why pleasure flights have to be called "trial lessons". All the people involved have a CPL already.
I don't think the law is clear. The PPL cost sharing rule is clear when you rent a plane for £90 wet and fly 3 mates and then if you pay £10 landing fee they can reimburse you £75 max.
It gets less obvious if you own the plane personally, and do a flight to see a customer of your own business. You then invoice your company for some money. Is the reimbursement under PPL Cost Sharing Rules ? You had no passengers!
Whatever you think the answer is to the above, let's assume you are a 100% shareholder (and a Director) of a ltd co X which owns the plane. You are also a 100% shareholder (and a Director) of another ltd. co Y which is your main business, and you do a flight to a see a customer of Y. It is actually OK for X to invoice Y for the full cost, in fact X can invoice Y anything it likes. And X doesn't invoice you personally for the flight at all. I have it from the CAA that this is OK, and it is certainly standard practice.