There is an excellent guide to writing a Constitution for your organisation on the website of the Resource Centre in Brighton and Hove:
List of charitable objects by the Charity Commission for your governing documents
https://www.gov.uk/government/publications/example-charitable-objects
Model constitutions for the various forms of charitable organisation are available on the Charity Commission website:
https://www.gov.uk/government/publications/setting-up-a-charity-model-governing-documents