(See also: housing benefit & council tax benefit; Crisis Loan information at the DWP website)
If you need money for a bond, ask one of the listed organisations if there is any help available.
If you find somewhere to live, but need money to pay rent in advance, you may be able to get a Crisis or Budgeting loan from the Social Fund. This will depend on your circumstances, as well as the type of benefit you claim. Apply at the Jobcentre Plus or ask one of the agencies for help.
Take your tenancy agreement, or a letter from the landlord stating the address, starting date, and the amount of rent needed in advance. You should usually get a decision that day.
When you move into a place you may also be able to get a Social Fund grant to help pay for things you need.
You cannot get Housing Benefit to pay rent in advance, but if you get a letter from the Housing Benefit office stating that you have applied, your landlord may be prepared to wait.