I did a charity flight last year. Suggest you have a look at AIC 25/2007 (White 136) which covers what you need to do. Lots of hoops to jump through and while the CAA appeared to be quite helpful when I first called them to discuss it, it became more tortuous as the process went on, with the final approval containing so many caveats and restrictions that it really wasn't a particularly worthwhile outing. Good luck and feel free to PM me if you'd like any further info.