Firstly, there's absolutely nothing to stop you leaving everything to your favourite charity simply by writing your own will. A suitable wording would be roughly as follows:
"This is the last will and testament of me, Desa Deleer, of 21 Acacia Avenue, Anytown, in the County of Nowtshire.
I hereby revoke all former Wills and testamentary dispositions made by me.
I appoint Cecil Smith, of 23 Acacia Avenue, Anytown in the County of Nowtshire, and Sybil Smith of the same address as my executors and trustees.
I devise and bequeath all my real and personal estate whatsoever and wheresoever situate unto my Trustees, upon trust, to sell, call in and convert the same into money, with power to postpone such sale, calling in and conversion for so long as they, in their absolute discretion, shall think fit, without being liable for loss.
My Trustees shall hold the net proceeds of sale and conversion of my ready money, upon trust, to pay thereout all my just debts, funeral and testamentary expenses.
Subject as aforesaid, my Trustees shall stand possessed of the residue of my estate, upon trust, to give in entirety to Cats Protection of the National Cat Centre, Chelwood Gate, Haywards Heath in the County of Sussex.
In witness whereof I, the said Desa Deleer, have to this my Last Will and Testament set my hand this seventh day of January in the year two thousand and twelve.
[Your signature]
Signed by the said Desa Deleer, the Testator, as and for his Last Will and Testament in the presence of us both at the same time who, at his request and in his presence and in the presence of each other, have hereunto subscribed our names as witnesses:
[Signatures of two witnesses]"
That's all there is to it. If, however, you insist ongoing to a charity which offers a free will-writing service, see these links:
http://www.internatio...0_iar-freewill-09.pdf
http://www.animalaid..../DONATE/legacy/ALL///
(There don't seem to be many animal charities which offer a free will-writing service but I've just done all of the work for you anyway!)
Chris