London to Cambridge (Tom Anderson route)

I believe this is the route from London to Cambridge described by Tom Anderson here: http://urchin.earth.li/~twic/Cycling_from_London_to_Cambridge.html

Cambridge to London (Matthew Belmonte route)

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.

London (north) to Cambridge

Cycle.Travel suggested route, which I have mostly ridden (from Nazeing north).

I recommend this route. It begins with canal towpaths and progresses to ever-lovelier countryside, before the flat, glorious run-up to Cambridge.

I have not ridden between the A110 (at the immediate start) to Nazeing, but this hopefully isn't as busy as the route suggested here: http://rides.tomtaylor.co.uk/routes/london-cambridge.html - which uses the incredibly busy A110, A112, and B114 all the way up to Nazeing Common (where I abandoned it). These roads have barely any shoulder, no cycling facilities, and very fast traffic - even though some aren't even B-roads.