Another thing is, if I want to go away with cadets on summer camp for a week, they're more likely to say yes if I'm only volunteering there, rather than having a proper paid contract aren't they?
They do sometimes molly, but quite often they don't - the whole point of volunteering is that it doesn't cost the charity or organisation anything, they can't afford it.
I'd be concerned about somewhere that wasn't a charity but was prepared to accept donations. Any organisation with charitable aims and exemptions with a turnover of more than £5k should register with the charity commission. If they are not a registered charity they may be a private limited company - in which case you have no guarantee that your donation won't just go on the manager's new BMW.