well, first you have to get the anti-social behaviour unit and the police to recognise that there is a problem with unwelcome visitors and give you there backing, then you have to get the approval of your neighbours (probably through you Tenants' and Residents' association - which you may have to set up yourself) and only then might Southwark start to look at finding the money to carry out the work. It took us five years! Sorry to be negative, but that's how it was for us!!
Jaybee is right, it is hard. However, I think you should ask your neighbours about the old doors, try to find out what went on before. Is there anything in your flat to show that there was once a door entry system?
You need to get your councillors involved, and set up a TRA whatever, you need one to get Southwark to listen to you on any issue.