The Bar Pro Bono Unit provides free advice and representation for eligible applicants. You'll need to complete their form and follow their procedures.
The Faculty of Advocates deals with all issues. You'll need a referral organisation, and a current list is found on their site.
The Free Representation Unit provides representation for employment and social security matters at tribunal. You'll need a referring organisation to make the referral to FRU on your behalf. There is a list of such organisations on their site.
The Disability Law Service (DLS) provides advice and representation for employment and community care matters in Greater London.
Shelter provide legal advice and attend court for people facing homelessness and eviction.
UNISON provide legal advice and representation for its members. You'll need to join, but once you do they will help you.
The Musicians' Union provide free legal advice and representation and much more. You'll need to join, but they offer a lot of help to its members.