Well, Geezer, let's take an example...Oxfam's administrative costs run at 11%. Reinvestment for expansion and, e.g., property upkeep, costs them another 10%. That's 21% in total. So does that leave only 79% of your donation to be spent directly on charitable works? Not necessarily...
For donations declared under the government's Gift Aid scheme, Oxfam can claim back the tax previously paid on your donation. So at basic tax rate of 20%, for evey £10 you donate, Oxfam will receive £12.50. Take off the 21% for admin and reinvestment from that £12.50, and you'll find that £9.875 (i.e. 98.75%) of your £10 remains, to be spent fully on charitable work.
Whatever charity you end up choosing, Coccinelle, make sure you apply to them for a Gift Aid form to fill in and forward so that they, and you, will get the best value from your donations.