Not in London but I'll see if Oxford and the rural surrounds have an equivalent page.
They do:
http://fixmystreet.oxfordshire.gov.uk/
@Oliver Schick started
London Fixed Gear and Single-Speed is a community of predominantly fixed gear and single-speed cyclists in and around London, UK.
This site is supported almost exclusively by donations. Please consider donating a small amount regularly.
They do:
http://fixmystreet.oxfordshire.gov.uk/