Surely the retailer is paying the manufacturer for them up-front anyway so it doesn't matter if we buy them or not - the deal has already been done so to speak?
Seriously though, if we all go out and buy one then the shop might get more in while the charity bars are still going
a) Not buy the choc and donate the full 35p to a charity box, or
b) buy a 15p chomp (or other cheap alternative) and donate 20p (the difference) to a charity box, or
c) buy whichever choc you like and donate a more generous sum to a charity box anyway.
A 5p donation isnt a reason to rush out and buy these, or to vote it hot IMO, as you're lining Cadburys pockets more than the charitys!