I looked into this some months ago, when asked if I would give a helicopter flight as a charity raffle prize. It's complicated, but not impossible, so long as you have the required number of hours, and can give the CAA at least 28 days notice (we didn't have time, so I couldn't do it; I'm promised to attempt to jump through the CAA hoops next year if they give me enough warning).
It seems unfair, but look at it from the CAA's point of view. If you take up friends, they know you are only a PPL, and they're prepared to take the risk. The general public doesn't know that, and expects the same...standards?...dunno the right word...as a commercial flight. So strictly speaking you should have a commercial licence and an AOC. But the CAA makes an exception for charity flights, if you have a fair amount of experience, let them know in good time, and tell them all the details. Still unfair? Possibly, but I can see their point of view.