I have some concerns about the advertising of seats on a public site such as this...
http://www.caa.co.uk/docs/1428/summa..._transport.pdf
Indicates that for
cost sharing (my italics) :
No advertising (except within a flying club operating the flight in which all adult persons carried must be a member).
(Page 7).
The same document covers the various options for charitable/non-commercial and commercial flight.