Sorry for the diversion from the thread, but I thought this one needed answering. Most overheads are fairly small, and the larger charities will often have breakdowns of where their money goes (including overheads). Without some overheads the money would not go to the right places so a necessary evil (if you can call it that).
There are some cases where the admin costs are unnecessary, such as Children In Need where you're better off giving directly to one of the supported charities (and the Give As You Earn scheme tbh), but the cost to the charities is outweighed by the donations they would not normally receive.