Can't find one.Why do you want an animal charity to draft the will ? If you have already got a will you can add a codicil to it to give a sum to charity.To take effect, the codicil has to be signed and witnessed and follow all the requirements of a full will, so you might as well have a new will done instead which includes such a gift.. It's better done that way because there's a risk that a codicil may get separated from the original will and not noticed.
Will writing services are all right for a very simple will provided they don't persuade you to make them the executors, because they'll charge your estate quite a lot for being executors.It's safer to have a properly qualified lawyer, a practising solicitor, to write the will.It's easy to leave a legacy in a fixed, named, sum to a charity.There's nothing special about it.