I have a gpx for a ride I have ridden between Upminster station and Cambridge you can have if you want.As above has anyone done this, and do you have a tried and trusted route, Organizing a club ride. all help appreciated.
Unless something has changed, I'd not do that unless you'll be entering London in the evening. Some of the roads are basic width and too busy for me during the day. Even blasting down the A10 would be preferable IMO.Cambridge to London, come down through Saffron Walden and Great Dunmow. Then pick up the reverse of the Dunwich Dynamo.
It depends why you're riding and what you want to see. Richard's Route is about 10 miles shorter and avoids most of the towns but has about a dozen climbs on it, whereas a mixed Byway/Sustrans route has three, out of Croydon, to Cranfield and to the Claydons - and the last of those isn't a big hill - but crossing Milton Keynes and Bedford can take time if you're not used to urban cycleway riding.