Not sure I understand what question you are asking....

But are you trying to get people to go for a 25 mile out and back overnight bike camping trip, for charity? What services do you provide to entice participants? I would think anyone who actually has the equipment to do a fully loaded/self-supported/camping bike tour (whatever you want to call it) would not be very interested in going only 25 miles a day, and the loaded bike touring crowd seems to pride themselves on both their self-sufficiency and their frugality. I'm not understanding your target market, or how you are getting people to do the ride.

I don't think you would make any money on that in the USA, you would get like zero participants. If it works in the UK, that would be interesting. Do you know of any events like that?