I was involved in setting a charity about 20 years back, and found an organisation called NCVO extremely helpful. (National Council for Voluntary Organisations).Basically they took our draft constitution and made sure it met the requirements for charitable status; then went on to successfully shepherd it through the process of registration with the Charities Commission.There was no charge.If they are still around, seek them out!
If you think that a charity requires a contract I am not sure that you are ready for this huge undertaking. A charity has to have either a constitution, a mem and arts or a trust deed.You dont need a solicitor to draw up a governing document for a charity. Your local CVS will do this for free. Google your town and CVS. If you have any problems making contact, email me.
