Also you can also use their site to plan direct or leisure routes - http://www.cyclemaps.com.
The Transport for London website includes a journey planner - http://journeyplanner.tfl.gov.uk. For journeys by bike, enter source and destination info, then choose "more options". Under "Use any of these modes of transport", tick "Cycle". Under "Additional cycling options" you may choose whether you want a cycle-only route or combined with public transport.