For cost sharing to be legal:
no information 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;
None of 'information', 'published', 'advertised', or 'flying club' are defined in the ANO!
I think the intention is perfectly clear -- the participants must understand the limitations of private flying and not believe that they are paying for a public transport operation. But intention is not necessarily important when it comes to legislation.