Go read Article 130 of the
ANO
(ii) (aa) no more than 4 persons (including the pilot) are carried on such a flight;
(bb) the proportion which such contribution bears to the total direct costs of the flight shall not exceed the proportion which the number of persons carried on the flight (excluding the pilot) bears to the number of persons carried on the flight (including the pilot);
(cc) no information concerning the flight shall have been published or advertised prior to the commencement of the flight other than, in the case of an aircraft operated by a flying club, advertising wholly within the premises of such a flying club in which case all the persons carried on such a flight who are aged 18 years or over shall be members of that flying club; and
(dd) no person acting as a pilot on such a flight shall be employed as a pilot by or be a party to a contract for the provision of services as a pilot with the operator of the aircraft being flown on the flight.
A strict interpretation of this means an Instructor breaks the law if he advertises a cost sharing jaunt to Le Touquet in his flying club and an owner or part owner breaks the law if he does likewise.