You're on the border of the Dulwich and Brixton Openreach exchanges, If your telephone number is 020 7, it's certainly Brixton, 020 8 is Dulwich. If you're using ADSL2+ which is copper broadband, it's going to be slow regardless as copper broadband loses speed the further away you are from the exchange. If you're on fibre (VDSL), you should be getting faster speeds as they have fibre cabinets which are closer to your home. For example residents on the East Dulwich Estate have 3MB on copper, but they get over 60MB download using fibre.