You can do cost sharing but then you are very limited on how the flight is advertised. Again, some grey areas in there, but basically the cost sharing limits mean it will never make anybody any money.
Detail on cost sharing is provided on Page 7 of the document above. This is the UK CAA Legal Department's interpretation of what is permitted when cost sharing:
a) Not more than four persons carried including the pilot.
b) Pilot must pay proportionate share of costs, e.g. if four persons carried he must pay 25%.
c) Passengers can meet their costs as they like, e.g. if three passengers carried one passenger can pay for all of them.
d) No advertising (except within a flying club operating the flight in which all adult persons carried must be a member).
Note item D which does allow advertising within a club environment.