The fact that a charity is involved in an agreement does not in itself mean that it was not intended to be legally binding. Many people enter into contracts with organised charities, supplying them with goods and services in the normal way, and such agreements are clearly intended to be legally enforceable.
It is different if the purpose of an agreement is charitable. Such agreements are often intended to be voluntary only, and the presumption is that they are not intended to be enforceable.