From the CAA web site:
Commercial Aviation
Commercial Aviation or public transport activities are those for which passengers pay a fare, or payment is made for cargo to be carried. If a passenger is asked to contribute in any way towards the costs of a flight, “valuable consideration”, the flight is likely to be for the purpose of public transport.
Some exceptions to this rule are:
* Some flights conducted to raise money for charities,
* Some flights where costs are shared between the pilot and up to three passengers,
* Some flights where the passengers are joint owners of the aircraft.
Please Note: The term “valuable consideration” can include methods of payment other than money, for example: free advertising or payment in kind.
Companies or individuals wishing to carry fare-paying passengers must hold an Air Operator Certificate (AOC).
An AOC is required for any public transport/commercial air transport flight.