Got a leaflet from Thames Water in the post this morning which had a weblink to check the water quality for your postcode -->Click Here
All the water quality info, hardness, fluoride etc; was from 2008, hardly up to date!
North Lambeth water has 284.5ppm (parts per million) which is hard.
Soft is up to 100ppm
Hard is 0ver 200ppm
And a Fluoride measure of 0.17ppm, don't know if that is average.