Whilst it is legal to advertise a flight for cost sharing to other members in your flying club, it is not legal to do the same to members of the public on Gumtree, how is this different?
I do not know the specifics in the UK, but in other countries and environments they circumvent the issue by selling "daily memberships" to clubs. Do you need to do anything in the UK to constitute a flying club, or does it exist immediately by declaration of intent?