I think SE23 postcodes have long been regarded as higher risk by the insurance companies. I remember when I moved here 20 years ago from W11 (Notting Hill/North Kensington) I had to pay an additional premium on my existing policy even though I needed no further coverage.

But surely this survey is flawed if it is only based on people getting new insurance quotes. I wouldn't be surprised if there are many areas with much higher burglary rates where people just don't bother with insurance or can't afford it.