Obtaining free ID to claim benefits

I will be soon volunteering at a local Salvation Army outlet helping clients with all aspects of their benefits etc. One of the main problem these clients have (mainly due to homeless) is having acceptable ID to claim benefits and open up simple banks accounts etc. Please can anyone advise me if there are any ways that ID can be obtained free of charge or with minimal payment especially with photo ID etc. Also what ID does the DWP have to accept which is free to obtain. Any ideas would be appreciated.