I don't think there is a problem with this at all, as long as you (as the pilot) pay the entire costs of the flight, and I believe the passengers should be known to you.
I have to admit that I do it a couple of times a year to raise funds for a couple of local charities. The more that is donated, the further we will go for lunch. The pax usually buy lunch and get the landing fees, but thats just good etiquette really. I should add that the flight generally happens a few months after the charitable donation.
Other forums have had similar queries raised, and the result seems to be it is fine, as long as the criteria above are met, and that you do not profit in any way. The CAA doc doesn't make reference to this type of scenario, but I did keep dozing off trying to read it.
