I believe this is the route described by Matthew Belmonte, here: http://www.mattababy.org/~belmonte/Home/Bicycle/Cambridge_to_London/

I haven't ridden (much) of this but I would especially advise, at the southern end, diverting onto NCN 1 via the canal / Lee towpath (see http://urchin.earth.li/~twic/Cycling_from_London_to_Cambridge.html), unless you feel that the A1010 is safe.