Looks like the answer is here!
http://www.sustrans.org.uk/what-we-do/national-cycle-network/route-numbering-system/route-6
@deleted 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.
Looks like the answer is here!
http://www.sustrans.org.uk/what-we-do/national-cycle-network/route-numbering-system/route-6