At just over 200km, this is my ‘go to’ route for an Audax “DIY” ride to keep me cycling over the winter months. It’s surprisingly low-traffic (once you get out of London), mostly good road surfaces and relatively flat. A nice easy winter 200k. There is an LNER train back to London which takes about 50 minutes (bike spaces must be booked) and a slightly slower Thameslink service which you can (in theory) just turn up and get on with your bike.
After crossing the Thames via Tower Bridge, the route uses the A10 and then the A1010 for a quick exit out of London. It is highly recommended to start early, to get this bit out of the way before the traffic builds up. A 5am start has usually worked for me, when there isn’t much on the road except buses and minicabs. If you can’t get away this early then there is the option for some of the A1010 of using the segregated cycle lane which runs from roughly where the route crosses the north circular to just shy of Waltham Cross. Or you could navigate your way to the River Lea towpath (follow CS3 after crossing Tower Bridge) and ride out of London on that before rejoining this route at Broxbourne (kilometre 35.7) or even Stansted St Margarets (kilometre 40.9). Once you’re out of London you can enjoy the next 170 kilometres of so on relatively quiet roads through Hertfordshire and Cambridgeshire.
London (Greenwich) to Peterborough – LBR version
The link above is to a GPX file which, at the time of publication, works on my Garmin Edge 530.