You can try Southwark law Centre ..Peckham (google it)
Or the Department of Trade and industry will give you a list, or victimn support has a list of locl firms for different aspects of the law. ( google VS at Borough Hight Street)
PS ACAS was also on Bourough High Street, not sure if they still are?
Heartily second Jan's suggestion of Huw Jones, he is brilliant. You can have for examply an hour with him to go through the pro's and con's of your case and he will guide you all the way if you cannot afford his services. Before you go if you make an appt you need to gather your contract of employment, grievance procedure, any letters / warnings, put them in order, and just draft on a sheet of paper the bones of your case [started on, promoted on, problem began on, action taken etc]
If you have car breakdown insurance or household insurance, have a quick look to see if you ticked the 'legal box' because you may find you are covered for an employment law case. Our old care breakdown insurance with Green Flag gave us access to a 24 hour advice line for generic advice on housing, personal injury, employment, a whole range of legal problems and up to £50k if problem was referred. Huw Jones is on the insurance panel and he is very good