Whilst Im all for donatin to charity, could it be possible that peeps can donate through choice like 110woman explained £25 is alot of money to some peeps, members wantin to go on the trip may have to save quite alot due to the distance (they maybe comin from afar) fuel costs are high at the moment and peeps may drop out due to the extra expense. Peeps that are travellin from afar, may also have campin fees, and if its a family weekend for them they may need to feed 3 kids for the weekend too... you add fuel costs, food for a family of 5, plus campin fees it can get quite expensive.
Maybe a donation tin at the end..... im sure peeps would feel more like donatin after a weekend of great company and fun lanin, plus experiencin the lovely scenery, they may just think ah yeah here's a tenner... a tenner is better than a member droppin out, yu will still raise a decent amount for charity that way.... family of 5, kids have eaten sweets, chocolate all day so wont want chips, put the chippie money in the pot instead.... that kinda thing.. a bit less formal.
Just my opinion.