I live in Balham just off Emanuel Road and provide the insurance for South London Lofts. I have never heard any negative reviews and they haven't had to register and insurance claim since I have worked with them. Craig Taylor, who is the MD there is also based in Balham.
From an insurance perspective you will need a specialist policy to cover your buildings if you are altering the structure (Assuming you are doing this as you are having a loft conversion). I can provide a product which can give you the correct cover for your existing buildings and additional contract works.
If you want to discuss this further my number is 02086437575 and email is email@example.com