Cost Sharing
If you are a PPL you can 'cost share'.
There must be no more than 4 persons (incl pilot) in the aircraft.
i.e. even total POB 5, with one not paying a penny, and you and others paying 1/4 is still illegal.
The flights cannot be advertised except in a flying club.
If you are in a club aeroplane the passengers must be over 18 and a member of the club (I guess day membership may count).
You also need to check the club flying order book and the ANO (article 107), as dufwer mentions