Charity Flights
What a lot of misconception!
Read the AIC bookworm pointed you at, 76/2003 (White 88). That contains a General Permission so if you comply with all the conditions in the AIC you do not need specific permission from the CAA, and it's legal.
If you want to do something which is outside the AIC, then speak to the CAA. They are very helpful. We go slightly outside the AIC with the dHMC Charity Weekend at Old Warden each June and it isn't a problem getting CAA approval.
Note that Charity Flights with passengers have to made from a licensed or Government aerodrome.
VA