The legal framework - The Homeless Persons Act 1977, the Housing Act 1996 and Homelessness Act 2002 - defines who is entitled to receive permanent housing by/on behalf of a local authority.
The main homelessness duty is owed to people who have a 'priority need' and are not 'intentionally homeless' (that is, judged as having made themselves homeless through their own actions).
Priority need groups include households with children and people who are vulnerable as a result of old age or mental or physical disability (set out in Homelessness Code of Guidance for Local Authorities). People not in these categories are only entitled to receive advice and assistance on how to find suitable accommodation.
http://england.shelter.org.uk/policy/policy-959.cfm